UPSC Prelims PYQs with explanations
Real questions from official papers, mapped to syllabus topics. Attempt each one; the answer and the reason appear instantly.
With reference to street-lighting, how do sodium lamps differ from LED lamps?
- 1. Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps.
- 2. As street-lights, sodium lamps have longer life span than LED lamps.
- 3. The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street-lighting.
Select the correct answer using the code given below.
Answer (c) & explanation
Statements 1 and 3 are correct (c). Statement 1 is right: a sodium lamp glows in all directions (360°), so much light is wasted unless reflectors are used, while LEDs are directional. Statement 3 is right: sodium lamps give a nearly monochromatic yellow light, so colours are hard to tell apart, while LEDs render colours far better. Statement 2 is wrong: LEDs last longer than sodium lamps, often 50,000 hours or more, and use much less power.
Notice India's programmes: UJALA (2015) for LED bulbs in homes, and the Street Lighting National Programme (SLNP), run by EESL, which replaced over 1.3 crore street lights with LEDs. The 2014 Nobel Prize in Physics went for the blue LED.
Answer as per the official UPSC answer key.
Read the passage
Researchers simulated street lighting on artificial grassland plots containing pea-aphids, sap-sucking insects, at night. These were exposed to two different types of light — a white light similar to newer commercial LED lights and an amber light similar to sodium street lamps. The low intensity amber light was shown to inhibit, rather than induce, flowering in a wild plant of the pea family which is a source of food for the pea-aphids in grasslands. The number of aphids was also significantly suppressed under the light treatment due to the limited amount of food available.
Which one of the following statements best reflects the most critical inference that can be made from the passage given above ?
Answer (b) & explanation
The critical inference is that light pollution can have a lasting harmful effect on an ecosystem (b). Step 1: the passage says night lighting cut flowering in plants, and through the food chain it cut aphid numbers. One change spread to other living things: a knock-on effect on the ecosystem. That is (b). Step 2: option (a) compares low and high intensity light. The passage reports no such comparison. Step 3: option (c) compares white light with other colours; also not reported. Option (d) talks of "proper intensity", which the passage does not discuss.
Tip: in science passages, the inference usually widens the finding one step (here, from plants to the ecosystem). Options adding new comparisons are traps.
Answer as per the official UPSC answer key.
Read the passage
Inequality violates a basic democratic norm : the equal standing of citizens. Equality is a relation that obtains between persons in respect of some fundamental characteristic that they share in common. Equality is, morally speaking, a default principle. Therefore, persons should not be discriminated on grounds such as race, caste, gender, ethnicity, disability, or class. These features of human condition are morally irrelevant. The idea that one should treat persons with respect not only because some of these persons possess some special features or talent, for example skilled cricketers, gifted musicians, or literary giants, but because persons are human beings, is by now part of commonsense morality.
With reference to the above passage, the following assumptions have been made :
- 1. Equality is a prerequisite for people to participate in the multiple transactions of society from a position of confidence.
- 2. Occurrence of inequality is detrimental to the survival of democracy.
- 3. Equal standing of all citizens is an idea that cannot actually be realised even in a democracy.
- 4. Right to equality should be incorporated into our values and day-to-day political vocabulary.
Which of the above assumptions are valid ?
Answer (c) & explanation
Assumptions 1 and 4 are valid (c). Step 1: the passage calls the equal standing of citizens a basic democratic norm. People need it to take part in society with confidence (1). Step 2: it says respecting persons as human beings is now part of "commonsense morality". So equality should be in our values and everyday vocabulary (4). Step 3: assumption 2 talks of the survival of democracy; the passage only says inequality violates a norm. Assumption 3 says equality cannot be realised, which goes against the passage. So options (a), (b) and (d) fail.
Tip: "violates a norm" is not the same as "destroys the system". Do not let an option exaggerate the passage's claim.
Answer as per the official UPSC answer key.
In the series _b_a_ba_b_abab_aab; fill in the six blanks ( _ ) using one of the following given four choices such that the series follows a specific order.
Answer (d) & explanation
The blanks are filled by ababab (d). Step 1: put the letters a, b, a, b, a, b into the six blanks in order. Step 2: the series becomes abbaab abbaab abbaab. Step 3: the block "abbaab" repeats three times, so the series has a clear order. Options (a) bababa, (b) baabba and (c) bbaabb break this repeating pattern.
Tip: in letter-series questions, try each option and look for a block that repeats. The series length (18 here) often hints at the block size (6).
Answer as per the official UPSC answer key.
Why is there a concern about copper smelting plants?
- 1. They may release lethal quantities of carbon monoxide into environment.
- 2. The copper slag can cause the leaching of some heavy metals into environment.
- 3. They may release sulphur dioxide as a pollutant.
Select the correct answer using the code given below.
Answer (b) & explanation
Slag leaching and sulphur dioxide are the concerns (b). Statement 2 is right: the leftover copper slag can leach heavy metals such as arsenic and lead into soil and water. Statement 3 is right: most copper ores are sulphides, so smelting gives off sulphur dioxide, which causes acid rain and breathing problems. Statement 1 is wrong: carbon monoxide is not a major pollutant from copper smelters; it comes mostly from incomplete burning in vehicles.
Notice the Sterlite copper plant at Thoothukudi, Tamil Nadu, which was closed in 2018 after protests in which 13 people were killed in police firing. The Supreme Court upheld the closure in 2024. Smelters often capture SO₂ to make sulphuric acid.
Answer as per the official UPSC answer key.
How is permaculture farming different from conventional chemical farming?
- 1. Permaculture farming discourages monocultural practices but in conventional chemical farming, monoculture practices are predominant.
- 2. Conventional chemical farming can cause increase in soil salinity but the occurrence of such phenomenon is not observed in permaculture farming.
- 3. Conventional chemical farming is easily possible in semi-arid regions but permaculture farming is not so easily possible in such regions.
- 4. Practice of mulching is very important in permaculture farming but not necessarily so in conventional chemical farming.
Select the correct answer using the code given below.
Answer (b) & explanation
Statements 1, 2 and 4 are correct (b). Permaculture designs a farm like a natural ecosystem, with many plants and animals working together and no chemicals. Statement 1 is right: it discourages monoculture, while chemical farming relies on it. Statement 2 is right: heavy irrigation and chemical fertilisers in conventional farming can raise soil salinity, which permaculture avoids. Statement 4 is right: mulching is central to permaculture, to keep moisture in and feed the soil. Statement 3 is wrong: permaculture suits dry and semi-arid areas well, because it saves water.
Notice that the term was coined by Bill Mollison and David Holmgren in 1970s Australia. Related Indian approaches include natural farming and Subhash Palekar's zero budget natural farming.
Answer as per the official UPSC answer key.
With reference to India, consider the following statements :
- 1. When a prisoner makes out a sufficient case, parole cannot be denied to such prisoner because it becomes a matter of his/her right.
- 2. State Governments have their own Prisoners Release on Parole Rules.
Which of the statements given above is/are correct?
Answer (b) & explanation
Only statement 2 is correct (b). Statement 1 is wrong: parole is a concession, not a right. The authority may refuse it even when grounds exist, for example if release would threaten public order. Statement 2 is right: prisons are a State subject (Entry 4, State List), so each State frames its own Prisoners Release on Parole Rules. Parole is a temporary release, for reasons such as a death or wedding in the family, and the time may or may not count towards the sentence.
Notice the difference from furlough, which is a periodic short release as a matter of good conduct, and from remission, which cuts the sentence. The Union issued the Model Prisons Act, 2023 for States to adopt.
Answer as per the official UPSC answer key.
Read the passage
At the heart of agroecology is the idea that agroecosystems should mimic the biodiversity levels and functioning of natural ecosystems. Such agricultural mimics, like their natural models, can be productive, pest-resistant, nutrient conserving, and resilient to shocks and stresses. In ecosystems there is no ‘waste’, nutrients are recycled indefinitely. Agroecology aims at closing nutrient loops, i.e., returning all nutrients that come out of the soil back to the soil such as through application of farmyard manure. It also harnesses natural processes to control pests and build soil fertility i.e., through intercropping. Agroecological practices include integrating trees with livestock and crops.
Consider the following :
- 1. Cover crops
- 2. Fertigation
- 3. Hydroponics
- 4. Mixed farming
- 5. Polyculture
- 6. Vertical farming
Which of the above farming practices can be compatible with agroecology, as implied by the passage ?
Answer (a) & explanation
Cover crops, mixed farming and polyculture fit agroecology (a). Step 1: agroecology works with natural soil and nutrient cycles. Cover crops return nutrients to the soil (1). Step 2: mixed farming joins crops and livestock, using dung as manure (4). Polyculture, growing many crops together, is intercropping (5). Step 3: fertigation (fertiliser through irrigation), hydroponics (plants in water) and vertical farming (indoor racks) bypass the soil cycle. So options (b), (c) and (d) fail.
Tip: agroecology means farming like an ecosystem. India's natural farming push, under the National Mission on Natural Farming (2024), follows these ideas.
Answer as per the official UPSC answer key.
Read the passage
In India, the objective of macroeconomic policy is to enhance the economic welfare of the people, and any one wing of such macro policy, monetary or fiscal, cannot independently work without active support of another.
Which one of the following statements best reflects the corollary to the passage given above ?
Answer (a) & explanation
The corollary is that the central bank cannot work independently of the government (a). Step 1: the passage says monetary policy and fiscal policy cannot work "without active support of another". Step 2: monetary policy is run by the central bank (the RBI in India). Fiscal policy (taxes and spending) is run by the government. So the RBI cannot work fully on its own. That is (a). Step 3: options (b) close regulation, (c) socialist policies and (d) financial sector reforms all bring in new ideas that the single sentence does not contain.
Tip: a corollary is a direct, small step from the passage. Translate the key terms (monetary = central bank, fiscal = government) and the answer appears.
Answer as per the official UPSC answer key.
The difference between a 2-digit number and the number obtained by interchanging the positions of the digits is 54.
Consider the following statements :
- 1. The sum of the two digits of the number can be determined only if the product of the two digits is known.
- 2. The difference between the two digits of the number can be determined.
Which of the above statements is/are correct ?
Answer (b) & explanation
1. Swapping the two digits of a number always changes it by 9 × (the gap between the digits). Example: 71 − 17 = 54 = 9 × 6.
2. So 54 ÷ 9 = 6: the digits differ by 6. Statement 2 is correct.
3. The number could be 71, 82 or 93, with digit sums 8, 10 and 12. The sum is not fixed.
4. Statement 1 says the sum can be found 'only if' the product is known. Other facts could also fix it, so 'only if' is wrong. Statement 1 is incorrect.
Answer as per the official UPSC answer key.
Which of the following are detritivores?
- 1. Earthworms
- 2. Jellyfish
- 3. Millipedes
- 4. Seahorses
- 5. Woodlice
Select the correct answer using the code given below.
Answer (c) & explanation
Earthworms, millipedes and woodlice are detritivores (c). Detritivores feed on detritus: dead plant and animal matter, such as fallen leaves and rotting wood. They break it into small pieces, which helps decomposers like bacteria and fungi finish the job and return nutrients to the soil. Item 1 is right: earthworms eat dead matter in soil. Item 3 is right: millipedes eat decaying leaves on the forest floor. Item 5 is right: woodlice are small crustaceans that eat rotting wood and leaves. Items 2 and 4 are wrong: jellyfish and seahorses are predators that catch small animals.
Notice the difference: detritivores eat detritus, while decomposers break it down chemically. Notice vermicomposting, which uses earthworms to make compost.
Answer as per the official UPSC answer key.
With reference to the water on the planet Earth, consider the following statements :
- 1. The amount of water in the rivers and lakes is more than the amount of groundwater.
- 2. The amount of water in polar ice caps and glaciers is more than the amount of groundwater.
Which of the statements given above is/are correct?
Answer (b) & explanation
Only statement 2 is correct (b). About 97% of Earth's water is salty, in the oceans. Of the 2.5–3% that is fresh, roughly 69% is locked in ice caps and glaciers, about 30% is groundwater, and only about 1% is surface water in lakes, rivers and swamps. Statement 1 is wrong: rivers and lakes hold far less water than groundwater. Statement 2 is right: ice caps and glaciers hold more water than groundwater.
Notice that Antarctica holds most of the world's ice. India has about 18% of the world's population but only about 4% of its freshwater. India is the largest user of groundwater in the world.
Answer as per the official UPSC answer key.
Consider the following statements :
- Statement 1 : The United Nations Capital Development Fund (UNCDF) and the Arbor Day Foundation have recently recognized Hyderabad as 2020 Tree City of the World.
- Statement 2 : Hyderabad was selected for the recognition for a year following its commitment to grow and maintain the urban forests.
Which one of the following is correct in respect of the above statements?
Answer (d) & explanation
Statement 1 is wrong, statement 2 is right (d). Statement 1 is wrong: Hyderabad was named a Tree City of the World for 2020, but the programme is run by the Food and Agriculture Organization (FAO) with the Arbor Day Foundation, not the UNCDF. Statement 2 is right: Hyderabad was recognised for its commitment to growing and maintaining urban trees and forests, helped by the Telangana government's Haritha Haram greening programme.
Notice the five standards for the title: assigning responsibility for trees, setting rules, keeping an inventory, allotting resources, and holding a celebration of trees. Mumbai later joined the list. Notice too the Nagar Van Yojana (2020), which aims to create urban forests in Indian cities.
the city and year are correct; only the UN body is swapped.
Answer as per the official UPSC answer key.
Read the passage
Policy makers and media have placed the blame for skyrocketing food prices on a variety of factors, including high fuel prices, bad weather in key food producing countries, and the diversion of land to non-food production. Increased emphasis, however, has been placed on a surge in demand for food from the most populous emerging economies. It seems highly probable that mass consumption in these countries could be well poised to create a food crisis.
With reference to the above passage, the following assumptions have been made :
- 1. Oil producing countries are one of the reasons for high food prices.
- 2. If there is a food crisis in the world in the near future, it will be in the emerging economies.
Which of the above assumptions is/are valid ?
Answer (d) & explanation
Neither assumption is valid (d). Step 1: assumption 1 blames oil-producing countries. But the passage blames high fuel prices, not the countries that produce oil. So 1 fails. Step 2: assumption 2 says a food crisis will be in emerging economies. The passage says demand from emerging economies may cause a crisis, but it does not say where it will strike. So 2 fails. Step 3: so options (a), (b) and (c) fail.
Tip: separate cause from location. "Emerging economies may create a crisis" is not "the crisis will happen in emerging economies". Small word shifts like this decide many CSAT answers.
Answer as per the official UPSC answer key.
A person P asks one of his three friends X as to how much money he had. X replied, "If Y gives me ₹ 40, then Y will have half of as much as Z, but if Z gives me ₹ 40, then three of us will have equal amount." What is the total amount of money that X, Y and Z have ?
Answer (b) & explanation
The total is ₹ 360 (b). Step 1: use the second condition first. If Z gives X ₹ 40, all three are equal. So X + 40 = Y = Z − 40. Step 2: the first condition: if Y gives X ₹ 40, Y has half of Z. So Y − 40 = Z ÷ 2. Put Y = Z − 40: Z − 80 = Z ÷ 2, so Z = 160. Step 3: then Y = 120 and X = 80. Total = 160 + 120 + 80 = ₹ 360. Options (a), (c) and (d) do not satisfy both conditions.
Tip: start with the condition that gives the simplest link ("all equal"). Then plug the answer options back in to check quickly.
Answer as per the official UPSC answer key.
Consider the following :
- 1. Foreign currency convertible bonds
- 2. Foreign institutional investment with certain conditions
- 3. Global depository receipts
- 4. Non-resident external deposits
Which of the above can be included in Foreign Direct Investments?
Answer (a) & explanation
FCCBs, FII investment with conditions, and GDRs can count as FDI (a). FDI means a lasting stake in a company. Item 1 is right: Foreign Currency Convertible Bonds turn into equity when converted, so they are counted as FDI under the rules. Item 2 is right: a foreign investor holding 10% or more of a listed company is treated as FDI, not portfolio investment. Item 3 is right: Global Depository Receipts represent shares of an Indian company held abroad. Item 4 is wrong: NRE deposits are bank deposits by non-resident Indians, a debt flow, not investment in a company.
Notice ADRs, the US-listed version of GDRs. The Arvind Mayaram Committee (2014) set the 10% line between FDI and FPI.
NRE deposits come from abroad and are large, but they are deposits, not ownership.
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. St. Francis Xavier was one of the founding members of the Jesuit Order.
- 2. St. Francis Xavier died in Goa and a church is dedicated to him there.
- 3. The Feast of St. Francis Xavier is celebrated in Goa each year.
Which of the statements given above are correct?
Answer (c) & explanation
Statements 1 and 3 are correct (c). Statement 1 is right: Francis Xavier was one of the founders of the Society of Jesus (Jesuits), with Ignatius of Loyola, in 1534. Statement 2 is wrong: he died in 1552 on Shangchuan island off China, not in Goa. His body was later brought to Goa and lies in the Basilica of Bom Jesus, which is dedicated to the Infant Jesus, not to him. Statement 3 is right: his feast falls on 3 December each year and draws large crowds in Goa.
Notice that the Churches and Convents of Goa, including Bom Jesus, are a UNESCO World Heritage Site. His relics are shown publicly about every ten years in the Exposition.
Answer as per the official UPSC answer key.
The term ‘ACE2’ is talked about in the context of
Answer (d) & explanation
ACE2 relates to the spread of viral diseases (d). ACE2 (angiotensin-converting enzyme 2) is a protein on the surface of cells in the lungs, gut, heart and blood vessels. The spike protein of SARS-CoV-2 binds to ACE2 to enter cells, like a key in a lock. That is why it became central to the study of how COVID-19 spreads and harms the body. It is not a gene in GM plants (a), part of NavIC (b) or radio collars (c).
Notice that the earlier SARS virus (2002–03) also used ACE2. Normally, ACE2 helps control blood pressure. Many vaccines target the spike protein to stop it binding to ACE2.
Answer as per the official UPSC answer key.
Read the passage
Approximately 80 percent of all flowering plant species are pollinated by animals, including birds and mammals, but the main pollinators are insects. Pollination is responsible for providing us with a wide variety of food, as well as many plant-derived medicines. At least one-third of the world’s agricultural crops depend upon pollination. Bees are the most dominant taxa when it comes to pollination and they are crucial to more than four hundred crops. Pollination is an essential service that is the result of intricate relationships between plants and animals, and the reduction or loss of either affects the survival of both. Effective pollination requires resources, such as refuges of pristine natural vegetation.
On the basis of the passage given above, the following assumptions have been made :
- 1. Sustainable production of India’s cereal food grains is impossible without the diversity of pollinating animals.
- 2. Monoculture of horticultural crops hampers the survival of insects.
- 3. Pollinators become scarce in cultivated areas devoid of natural vegetation.
- 4. Diversity in insects induces diversity of plants.
Which of the above assumptions is/are valid ?
Answer (d) & explanation
Assumptions 3 and 4 are valid (d). Step 1: the passage says pollination needs "refuges of pristine natural vegetation". So pollinators become scarce in farms with no natural plants (3). Step 2: it says losing one partner affects the other, so the diversity of insects and plants go together (4). Step 3: assumption 1 is about cereal grains, and assumption 2 about monoculture of horticulture crops. Neither is mentioned. Also, most cereals like rice and wheat are wind- or self-pollinated. So options (a), (b) and (c) fail.
Tip: reject any statement whose key term (cereals, monoculture) never appears in the passage. Pick the ones you can trace to a line.
Answer as per the official UPSC answer key.
Read the passage
Aristocratic government ruins itself by limiting too narrowly the circle within which power is confined; oligarchic government ruins itself by the incautious scramble for immediate wealth. But even democracy ruins itself by excess of democracy. Its basic principle is the equal right of all to hold office and determine public policy. This is, at first glance, a delightful arrangement; it becomes disastrous because the people are not properly equipped by education to select the best rulers and the wisest courses. The people have no understanding and only repeat what their rulers are pleased to tell them. Such a democracy is tyranny or autocracy. — Plato
Which one of the following statements best reflects the crux of the passage given above ?
Answer (b) & explanation
The crux is that any form of government tends to decay through an excess of its basic principle (b). Step 1: the passage shows aristocracy ruins itself by keeping power too narrow, oligarchy by greed for wealth, and democracy by "excess of democracy". Each fails by pushing its own principle too far. That is (b). Step 2: option (a), societies trying different governments, is only background. Step 3: option (c) says education ensures a perfect democracy, which overpromises. Option (d) calls government a "necessary evil"; Plato does not argue this.
Tip: when a passage gives several examples of one pattern, the crux is the pattern itself, not any single example.
Answer as per the official UPSC answer key.