UPSC Prelims PYQs with explanations
Real questions from official papers, mapped to syllabus topics. Attempt each one; the answer and the reason appear instantly.
When a certain number is multiplied by 7, the product entirely comprises ones only (1111...). What is the smallest such number ?
Answer (d) & explanation
The smallest number is 15873 (d). Step 1: multiply each option by 7 and look for an answer made only of ones. Step 2: 15873 × 7 = 111111. That works. Step 3: the others fail: 15713 × 7 = 109991, 15723 × 7 = 110061 and 15783 × 7 = 110481. Also, 111111 is the smallest all-ones number divisible by 7 (1, 11, 111, 1111 and 11111 are not). So (d).
Tip: 111111 = 3 × 7 × 11 × 13 × 37. It is a handy number: divisible by 7, 11 and 13 together.
Answer as per the official UPSC answer key.
With reference to Chausath Yogini Temple situated near Morena, consider the following statements :
- 1. It is a circular temple built during the reign of Kachchhapaghata Dynasty.
- 2. It is the only circular temple built in India.
- 3. It was meant to promote the Vaishnava cult in the region.
- 4. Its design has given rise to a popular belief that it was the inspiration behind the Indian Parliament building.
Which of the statements given above are correct?
Answer (c) & explanation
Statements 1 and 4 are correct (c). Statement 1 is right: the Chausath Yogini temple at Mitaoli, near Morena in Madhya Pradesh, was built around 1323 under the Kachchhapaghata kings. It is a circular ring of 64 cells around a central Shiva shrine. Statement 2 is wrong: other circular yogini temples exist, at Hirapur and Ranipur-Jharial in Odisha. Statement 3 is wrong: yogini temples belong to Tantric Shakta-Shaiva worship, not the Vaishnava cult. Statement 4 is right: its round plan is popularly said to have inspired the old Parliament House, designed by Edwin Lutyens and Herbert Baker.
Notice that the old Parliament House is now called Samvidhan Sadan, after the new building opened in 2023.
Answer as per the official UPSC answer key.
With reference to recent developments regarding ‘Recombinant Vector Vaccines’, consider the following statements :
- 1. Genetic engineering is applied in the development of these vaccines.
- 2. Bacteria and viruses are used as vectors.
Which of the statements given above is/are correct?
Answer (c) & explanation
Both statements are correct (c). A recombinant vector vaccine puts a gene for a pathogen's antigen into a harmless carrier, which then makes the antigen inside the body and trains the immune system. Statement 1 is right: making the vaccine relies on genetic engineering. Statement 2 is right: the carrier can be a virus, such as the adenovirus in Covishield and Sputnik V, or a weakened bacterium, such as Salmonella or BCG.
Notice other COVID vaccine types: Covaxin used a whole inactivated virus; Pfizer and Moderna used mRNA; ZyCoV-D was the world's first DNA vaccine for people; Corbevax was a protein subunit vaccine. India also approved an intranasal vaccine (iNCOVACC).
Answer as per the official UPSC answer key.
Consider the following statements in respect of the ICC World Test Championship :
- 1. The finalists were decided by the number of matches they won.
- 2. New Zealand was ranked ahead of England because it won more matches than England.
Which of the above statements is/are correct?
Answer (d) & explanation
Neither statement is correct (d). Statement 1 is wrong: for the first ICC World Test Championship (2019–21), many series were cancelled because of COVID, so finalists were chosen by the percentage of points earned out of points contested, not by matches won. Statement 2 is wrong: New Zealand was placed ahead of England on that percentage, not because it won more matches. New Zealand then beat India in the final at Southampton in June 2021.
Notice the later winners: Australia beat India at The Oval in 2023, and South Africa beat Australia at Lord's in 2025. India has reached the final twice, in 2021 and 2023. The WTC was set up to give Test cricket a context and a champion.
Answer as per the official UPSC answer key.
Replace the incorrect term by the correct term in the given sequence
3, 2, 7, 4, 13, 10, 21, 18, 31, 28, 43, 40
where odd terms and even terms follow the same pattern.
Answer (a) & explanation
The incorrect term 2 should be replaced by 0 (a). Step 1: odd places: 3, 7, 13, 21, 31, 43. The gaps are 4, 6, 8, 10, 12. Step 2: even places: 2, 4, 10, 18, 28, 40. The gaps from 4 on are 6, 8, 10, 12, matching the odd places. So the first gap must also be 4. Step 3: the first even term should be 4 − 4 = 0, not 2. Options (b), (c) and (d) give a wrong first gap.
Tip: when a question says two sub-series follow the same pattern, find the pattern in the clean one, then fix the other. Each even term here is 3 less than its odd partner.
Answer as per the official UPSC answer key.
In an objective type test of 90 questions, 5 marks are allotted for every correct answer and 2 marks are deducted for every wrong answer. After attempting all the 90 questions, a student got a total of 387 marks. What is the number of incorrect responses ?
Answer (a) & explanation
The student gave 9 wrong answers (a). Step 1: if all 90 were right, the score would be 90 × 5 = 450. Step 2: each wrong answer loses the 5 marks it would have earned and costs 2 more. So each wrong answer reduces the score by 7. Step 3: the score fell by 450 − 387 = 63. So wrong answers = 63 ÷ 7 = 9. Check: 81 × 5 − 9 × 2 = 405 − 18 = 387. Options (b), (c) and (d) fail this check.
Tip: in negative-marking questions, assume all are correct, then divide the shortfall by (marks for right + penalty for wrong).
Answer as per the official UPSC answer key.
Which one of the following is likely to be the most inflationary in its effects?
Answer (d) & explanation
Creating new money is the most inflationary (d). Option (a) is wrong: repaying public debt returns money to lenders and does not add demand much. Options (b) and (c) are wrong: borrowing from the public or from banks only moves existing money to the government, so total money in the economy changes little. Option (d) is right: creating new money, where the central bank finances the deficit (called monetisation or deficit financing), adds fresh money without adding goods, so prices rise the most.
Notice that India ended automatic monetisation through ad hoc Treasury Bills in 1997 (agreement of 1994). The FRBM Act, 2003 barred the RBI from buying government bonds in primary auctions, with an escape clause used during COVID.
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. The Montagu-Chelmsford Reforms of 1919 recommended granting voting rights to all the women above the age of 21.
- 2. The Government of India Act of 1935 gave women reserved seats in legislature.
Which of the statements given above is/are correct?
Answer (b) & explanation
Only statement 2 is correct (b). Statement 1 is wrong: the Montagu-Chelmsford Reforms of 1919 did not give votes to all women above 21. They left women's franchise to the provincial councils. Madras went first in 1921, and only women with property or education qualified. Votes for all adult women came only with the Constitution in 1950. Statement 2 is right: the Government of India Act, 1935 reserved seats for women in the legislatures and widened their franchise.
Notice women leaders of the time: Sarojini Naidu, first Indian woman Congress president (1925), and Muthulakshmi Reddy, first woman legislator (Madras, 1927). The 106th Amendment (2023) reserves one-third of Lok Sabha and Assembly seats for women.
Answer as per the official UPSC answer key.
Which one of the following is a reason why astronomical distances are measured in light-years?
Answer (d) & explanation
Light-years work because the speed of light is constant (d). A light-year is the distance light covers in one year in a vacuum, about 9.46 × 10¹² km. It can serve as a unit only because the speed of light in a vacuum is always the same, about 3 × 10⁵ km per second, for every observer. Option (a) is wrong: distances between stars and galaxies do change; the universe is even expanding. Option (b) is wrong: gravity of stellar bodies changes as they evolve. Option (c) is wrong: light does not always travel straight; gravity bends it (gravitational lensing).
Notice the other units: the astronomical unit (Earth–Sun distance, about 150 million km) and the parsec (about 3.26 light-years). The nearest star after the Sun, Proxima Centauri, is about 4.2 light-years away.
Answer as per the official UPSC answer key.
If 32019 is divided by 10, then what is the remainder ?
Answer (c) & explanation
The remainder is 7 (c). Step 1: the remainder on dividing by 10 is just the unit digit. Step 2: powers of 3 end in a repeating cycle: 3¹ = 3, 3² = 9, 3³ = 27, 3⁴ = 81. The cycle is 3, 9, 7, 1, of length 4. Step 3: 2019 ÷ 4 leaves a remainder of 3 (2019 = 4 × 504 + 3). So take the 3rd term of the cycle: 7. Option (b), 3, is the first term. Options (a) and (d) are the 4th and 2nd terms.
Tip: unit digits of powers repeat every 4 (or fewer). Divide the power by 4 and use the remainder; a remainder of 0 means the 4th term.
Answer as per the official UPSC answer key.
There are two Classes A and B having 25 and 30 students respectively. In Class-A the highest score is 21 and lowest score is 17. In Class-B the highest score is 30 and lowest score is 22. Four students are shifted from Class-A to Class-B. Consider the following statements :
- 1. The average score of Class-B will definitely decrease.
- 2. The average score of Class-A will definitely increase.
Which of the above statements is/are correct ?
Answer (a) & explanation
Only statement 1 is correct (a). Step 1: every student in Class A scores between 17 and 21. Every student in Class B scores 22 or more. Step 2: the four new students in B all score less than anyone already in B. So B's average must fall (statement 1). Step 3: Class A's average depends on who leaves. If the four toppers leave, A's average falls. If the four lowest leave, it rises. So it does not "definitely" increase (statement 2 fails). Options (b), (c) and (d) fail.
Tip: adding values below the average always lowers it. Removing values changes the average only according to which ones are removed; "definitely" needs every case to agree.
the word "definitely". 2 can happen, but it does not have to.
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Answer as per the official UPSC answer key.
There are 6 persons arranged in a row. Another person has to shake hands with 3 of them so that he should not shake hands with two consecutive persons. In how many distinct possible combinations can the handshakes take place ?
Answer (b) & explanation
There are 4 ways (b). Step 1: number the people 1 to 6 in the row. We must choose 3 of them, with no two standing side by side. Step 2: list the choices: (1, 3, 5), (1, 3, 6), (1, 4, 6) and (2, 4, 6). Step 3: that is 4 ways. Options (a) 3, (c) 5 and (d) 6 miss or add cases.
Tip: a quick formula for choosing k items from n in a row with no two adjacent is C(n − k + 1, k). Here C(4, 3) = 4.
Answer as per the official UPSC answer key.
Which one of the following is used in preparing a natural mosquito repellent?
Answer (c) & explanation
Lemongrass is used in natural mosquito repellent (c). Lemongrass oil contains citronella-like compounds such as citral, which keep mosquitoes away, and it is used in creams, sprays and candles. Option (a) is wrong: Congress grass (Parthenium) is a harmful, invasive weed that causes allergies and skin problems. Option (b) is wrong: elephant grass is a tall grass grown as fodder and habitat for animals like rhinos. Option (d) is wrong: nut grass is a common weed of farms.
Notice that India is a major producer of lemongrass oil, and the CSIR Aroma Mission promotes it, especially in dry and degraded lands. Other natural repellents include neem and citronella oil.
Answer as per the official UPSC answer key.
Consider the following rivers :
- 1. Brahmani
- 2. Nagavali
- 3. Subarnarekha
- 4. Vamsadhara
Which of the above rise from the Eastern Ghats?
Answer (b) & explanation
The Nagavali and Vamsadhara rise from the Eastern Ghats (b). Item 2 is right: the Nagavali rises in the Eastern Ghats of Kalahandi district, Odisha, and reaches the Bay of Bengal near Srikakulam. Item 4 is right: the Vamsadhara also rises in the Eastern Ghats of southern Odisha and enters the sea at Kalingapatnam in Andhra Pradesh. Item 1 is wrong: the Brahmani, formed by the South Koel and Sankh, rises on the Chota Nagpur plateau. Item 3 is wrong: the Subarnarekha also rises on the Chota Nagpur plateau near Ranchi.
Notice the Vamsadhara River Water Disputes Tribunal, which settled the Andhra–Odisha dispute. Other Eastern Ghats rivers include the Rushikulya, known for olive ridley turtle nesting.
Answer as per the official UPSC answer key.
With reference to India, consider the following statements :
- 1. There is only one citizenship and one domicile.
- 2. A citizen by birth only can become the Head of State.
- 3. A foreigner once granted the citizenship cannot be deprived of it under any circumstances.
Which of the statements given above is/are correct?
Answer (a) & explanation
Only statement 1 is correct (a). Statement 1 is right: India has single citizenship, and the Supreme Court (in Pradeep Jain, 1984) has said there is only one domicile, that of India, not of each State. Statement 2 is wrong: Article 58 needs the President to be a citizen, not a citizen by birth; naturalised citizens are eligible too, unlike in the US. Statement 3 is wrong: under Section 10 of the Citizenship Act, 1955, a naturalised or registered citizen can be deprived of citizenship, for example for fraud or disloyalty.
Notice the ways to gain citizenship: birth, descent, registration, naturalisation and incorporation of territory. The Citizenship (Amendment) Act, 2019, with rules notified in 2024, eased naturalisation for some groups.
statement 2 borrows the US rule (natural-born citizen) and applies it to India.
Answer as per the official UPSC answer key.
A boy plays with a ball and he drops it from a height of 1·5 m. Every time the ball hits the ground, it bounces back to attain a height 4/5th of the previous height. The ball does not bounce further if the previous height is less than 50 cm. What is the number of times the ball hits the ground before the ball stops bouncing ?
Answer (b) & explanation
The ball hits the ground 5 times (b). Step 1: each bounce reaches 4/5 of the previous height. It starts at 150 cm. Step 2: hit 1 → rises to 120 cm. Hit 2 → 96 cm. Hit 3 → about 77 cm. Hit 4 → about 61 cm. Hit 5 → about 49 cm. Step 3: after the fifth hit, the height is below 50 cm, so it does not bounce again. The ball hits the ground 5 times. Option (a), 4, stops too early. Options (c) and (d) go too far.
Tip: list each step in a small table. Converting metres to centimetres first (1·5 m = 150 cm) avoids decimal errors.
Some readers count one more landing after the last small bounce; the official answer is (b), as printed.
Answer as per the official UPSC answer key.
Which day is 10th October, 2027 ?
Answer (a) & explanation
10 October 2027 is a Sunday (a). Step 1: use an anchor date: 10 October 2021 was a Sunday (the CSAT exam was held that day). Step 2: count odd days from 10 Oct 2021 to 10 Oct 2027. That is 6 years, and each normal year adds 1 odd day. Only one 29 February falls in between (2024), adding 1 more. Total = 6 + 1 = 7 odd days = 0 extra. Step 3: so the weekday stays the same: Sunday. Options (b), (c) and (d) come from missing or adding the leap day.
Tip: a normal year shifts the weekday by 1, a leap year by 2. Count only the 29 Februarys that fall inside your range.
Answer as per the official UPSC answer key.
A man completes 7/8 of a job in 21 days. How many more days will it take him to finish the job if quantum of work is further increased by 50% ?
Answer (d) & explanation
He needs 15 more days (d). Step 1: he does 7/8 of the job in 21 days. So the whole job takes 21 ÷ 7/8 = 24 days. Step 2: the work is raised by 50%, so the total is now 1.5 jobs. Work left = 1.5 − 7/8 = 12/8 − 7/8 = 5/8 of a job. Step 3: 5/8 of a job takes 5/8 × 24 = 15 days. Option (a), 24, is the time for one full job. Options (b) and (c) miscalculate the remaining work.
Tip: first find the time for one full job. Then any amount of work is just that fraction of the time.
the 50% is added to the whole job, not to the remaining 1/8.
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Answer as per the official UPSC answer key.
Which one of the following ancient towns is well-known for its elaborate system of water harvesting and management by building a series of dams and channelizing water into connected reservoirs?
Answer (a) & explanation
Dholavira is famous for its water management (a). Dholavira, on Khadir island in the Rann of Kachchh, Gujarat, stood between two seasonal streams, the Mansar and Manhar. Its people built dams across them and cut huge stone reservoirs along the city walls to store monsoon water in a dry land. Kalibangan (b) in Rajasthan is known for a ploughed field and fire altars. Rakhigarhi (c) in Haryana is among the largest Harappan sites. Ropar (d) in Punjab is known for a burial of a dog with a man.
Notice that Dholavira became a UNESCO World Heritage Site in 2021. It also has a signboard of ten large Harappan letters. Lothal in Gujarat had a dockyard.
Answer as per the official UPSC answer key.
In the context of hereditary diseases, consider the following statements :
- 1. Passing on mitochondrial diseases from parent to child can be prevented by mitochondrial replacement therapy either before or after in vitro fertilization of egg.
- 2. A child inherits mitochondrial diseases entirely from mother and not from father.
Which of the statements given above is/are correct?
Answer (c) & explanation
Both statements are correct (c). Statement 2 is right: mitochondria come only from the egg, because the sperm's mitochondria are destroyed after fertilisation. So mitochondrial DNA diseases pass down entirely from the mother. Statement 1 is right: mitochondrial replacement therapy swaps the mother's faulty mitochondria for healthy donor mitochondria. This can be done before fertilisation (spindle transfer) or after it (pronuclear transfer), with in vitro fertilisation. The child then carries DNA from three people, hence "three-parent babies".
Notice that the UK legalised the method in 2015, and in 2025 reported eight healthy babies born this way. Mitochondria are the cell's powerhouses, making energy as ATP.
Answer as per the official UPSC answer key.