Science and Technology: previous-year questions
185 real UPSC Prelims questions (2011–2026) with the official answer and a clear explanation. 50 of them came in the last five papers.
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With reference to ‘fuel cells’ in which hydrogen-rich fuel and oxygen are used to generate electricity, consider the following statements :
- 1. If pure hydrogen is used as a fuel, the fuel cell emits heat and water as by-products.
- 2. Fuel cells can be used for powering buildings and not for small devices like laptop computers.
- 3. Fuel cells produce electricity in the form of Alternating Current (AC).
Which of the statements given above is/are correct?
Answer (a) & explanation
Only statement 1 is correct (a). A hydrogen fuel cell combines hydrogen and oxygen electrochemically and gives off only water and heat (1). Statement 2 is wrong: fuel cells come in all sizes, from micro cells for laptops to megawatt units for buildings. Statement 3 is wrong: their output is direct current, which an inverter converts to AC.
Fuel cells are clean at the point of use, but their green credentials depend on how the hydrogen is made. India's National Green Hydrogen Mission (2023) aims for 5 million tonnes a year by 2030. Notice how this links chemistry to clean energy policy.
Answer as per the official UPSC answer key.
With reference to the Indian Renewable Energy Development Agency Limited (IREDA), which of the following statements is/are correct?
- 1. It is a Public Limited Government Company.
- 2. It is a Non-Banking Financial Company.
Select the correct answer using the code given below.
Answer (c) & explanation
Both are correct (c). IREDA (1987), under the Ministry of New and Renewable Energy, is a public limited government company (1). It is registered with the RBI as a Non-Banking Financial Company that lends to renewable energy and energy-efficiency projects (2).
IREDA was listed on the stock exchanges in 2023 and became a Navratna company in 2024. It helps finance India's goal of 500 GW of non-fossil capacity by 2030. Notice how specialised lenders bridge the gap between policy targets and actual projects on the ground.
Answer as per the official UPSC answer key.
With reference to the use of nano-technology in health sector, which of the following statements is/are correct?
- 1. Targeted drug delivery is made possible by nanotechnology.
- 2. Nanotechnology can largely contribute to gene therapy.
Select the correct answer using the code given below.
Answer (c) & explanation
Both are correct (c). Nanoparticles such as liposomes, dendrimers and gold particles can carry drugs straight to diseased cells like tumours, sparing healthy tissue (1). They also act as non-viral carriers of genes in gene therapy (2).
The lipid nanoparticles used in mRNA COVID-19 vaccines apply the same idea, wrapping fragile genetic material to carry it into cells. Nano-based diagnostics can also detect diseases earlier. Notice how a physics idea, the special behaviour of matter at the nanoscale, is reshaping medicine, though safety and toxicity questions remain.
Answer as per the official UPSC answer key.
In the context of modern scientific research, consider the following statements about ‘IceCube’, a particle detector located at South Pole, which was recently in the news :
- 1. It is the world’s largest neutrino detector, encompassing a cubic kilometre of ice.
- 2. It is a powerful telescope to search for dark matter.
- 3. It is buried deep in the ice.
Which of the statements given above is/are correct?
Answer (d) & explanation
All three are correct (d). IceCube, at the Amundsen–Scott South Pole station, has thousands of light sensors spread through a cubic kilometre of ice, making it the world's largest neutrino detector (1). It also searches for dark matter particles (2). Its sensors are buried 1.5–2.5 km deep in the ice (3), where the ice is very clear and shielded from other radiation.
Neutrinos are nearly massless particles that pass through matter almost untouched, so they carry information from distant cosmic events. India's proposed India-based Neutrino Observatory planned to study them under a mountain in Tamil Nadu.
Answer as per the official UPSC answer key.
With reference to ‘Near Field Communication (NFC) Technology’, which of the following statements is/are correct?
- 1. It is a contactless communication technology that uses electromagnetic radio fields.
- 2. NFC is designed for use by devices which can be at a distance of even a metre from each other.
- 3. NFC can use encryption when sending sensitive information.
Select the correct answer using the code given below.
Answer (c) & explanation
Statements 1 and 3 are correct (c). NFC is a contactless technology using electromagnetic radio fields at 13.56 MHz (1). It supports encryption for sensitive data such as tap-to-pay transactions (3). Statement 2 is wrong: NFC works only within about 4 cm, which is part of its security. A one-metre range is closer to some RFID or Bluetooth uses.
NFC powers contactless cards, phone wallets, metro cards and FASTag-like systems, though FASTag itself uses RFID. Notice how short range is a design choice for safety: a device must be held close on purpose.
Answer as per the official UPSC answer key.
The term ‘Goldilocks Zone’ is often seen in the news in the context of
Answer (c) & explanation
The Goldilocks zone, or habitable zone, is the band of orbits around a star where a planet is neither too hot nor too cold, so liquid water can exist. It is used in the search for Earth-like planets in outer space (c). It does not refer to habitable limits above Earth's surface (a), shale gas regions (b) or meteorites with precious metals (d).
The name comes from the fairy tale in which Goldilocks picks porridge that is "just right". Missions such as Kepler, TESS and the James Webb Space Telescope study such planets. Notice how astronomy meets biology in the search for life.
Answer as per the official UPSC answer key.
The Genetic Engineering Appraisal Committee is constituted under the
Answer (c) & explanation
The Genetic Engineering Appraisal Committee works under the Rules, 1989 framed under the Environment (Protection) Act, 1986 (c). It sits in the Environment Ministry and approves the environmental release of GM organisms. The Food Safety Act (a), the GI Act (b) and the Wildlife Act (d) do not create it.
GEAC approved Bt cotton in 2002, India's only commercial GM crop so far, and conditionally approved GM mustard (DMH-11) for environmental release in 2022. It also cleared then put on hold Bt brinjal in 2009–10. Notice how biotech regulation balances innovation with biosafety.
Answer as per the official UPSC answer key.
Which of the following pairs is/are correctly matched?
| Spacecraft | Purpose
| 1. Cassini-Huygens | Orbiting the Venus and transmitting data to the Earth
| 2. Messenger | Mapping and investigating the Mercury
| 3. Voyager 1 and 2 | Exploring the outer solar system
Select the correct answer using the code given below.
Answer (b) & explanation
Pairs 2 and 3 are correct (b). Cassini–Huygens studied Saturn, not Venus, and its Huygens probe landed on the moon Titan in 2005, so pair 1 is wrong. NASA's MESSENGER orbited and mapped Mercury from 2011 to 2015. Voyager 1 and 2, launched in 1977, explored the outer planets and have since entered interstellar space.
Venus missions include Magellan, ESA's Venus Express and Japan's Akatsuki; India plans its Venus Orbiter Mission. Notice how UPSC likes pairing spacecraft with their targets: a short table of missions saves marks.
Answer as per the official UPSC answer key.
Which of the following is/are the example/examples of chemical change?
- 1. Crystallization of sodium chloride
- 2. Melting of ice
- 3. Souring of milk
Select the correct answer using the code given below.
Answer (b) & explanation
Only the souring of milk is a chemical change (b): bacteria turn lactose into lactic acid, which curdles the milk protein casein, and the change cannot be reversed. Crystallisation of sodium chloride (1) and melting of ice (2) change only the physical state; the substance stays the same and the change can be undone.
Signs of a chemical change include a new substance, a change in colour or smell, gas given off, or heat released or absorbed. Rusting, burning and digestion are other examples. Notice how curd-making is everyday microbiology.
Answer as per the official UPSC answer key.
Consider the following techniques/phenomena :
- 1. Budding and grafting in fruit plants
- 2. Cytoplasmic male sterility
- 3. Gene silencing
Which of the above is/are used to create transgenic crops?
Answer (b) & explanation
Transgenic crops carry genes from another species. Cytoplasmic male sterility systems, such as GM mustard's barnase–barstar system, and gene silencing through RNAi, used in virus-resistant papaya and non-browning apples, are built in through genetic engineering. So 2 and 3 are correct (b). Budding and grafting are traditional propagation methods that join plant parts but change no genes, so options with 1 fail.
India has approved only Bt cotton for commercial cultivation, while DMH-11 mustard got environmental release approval in 2022. Notice how biotechnology meets policy, trade and farmer concerns.
Answer as per the official UPSC answer key.
Among the following organisms, which one does not belong to the class of other three?
Answer (a) & explanation
Mites, scorpions and spiders are arachnids: they have eight legs, two main body parts and no antennae. The crab is a crustacean, with ten legs and two pairs of antennae, so it is the odd one out (a). All four belong to the phylum Arthropoda, with jointed legs and an exoskeleton.
Other arthropod classes include insects (six legs, three body parts, one pair of antennae) and myriapods such as centipedes. Notice how counting legs and antennae is often enough to classify an animal, a handy shortcut for biology questions.
Answer as per the official UPSC answer key.
With reference to technologies for solar power production, consider the following statements :
- 1. ‘Photovoltaics’ is a technology that generates electricity by direct conversion of light into electricity, while ‘Solar Thermal’ is a technology that utilizes the Sun’s rays to generate heat which is further used in electricity generation process.
- 2. Photovoltaics generates Alternating Current (AC), while Solar Thermal generates Direct Current (DC).
- 3. India has manufacturing base for Solar Thermal technology, but not for Photovoltaics.
Which of the statements given above is/are correct?
Answer (a) & explanation
Statement 1 is correct: photovoltaic cells convert light directly into electricity, while solar thermal (concentrated solar power) uses mirrors to heat a fluid whose steam drives a turbine. Statement 2 has it reversed: PV gives direct current (DC), which an inverter turns into AC, while a turbine-driven generator produces AC. Statement 3 is wrong because in 2014 India already had a PV manufacturing base for cells and modules, though small.
Since then the PLI scheme and the ALMM list have pushed domestic module making. Notice how a basic physics idea (AC vs DC) turns into an energy-policy question.
Answer as per the official UPSC answer key.
Consider the following pairs :
| Vitamin | Deficiency disease
| 1. Vitamin C | Scurvy
| 2. Vitamin D | Rickets
| 3. Vitamin E | Night blindness
Which of the pairs given above is/are correctly matched?
Answer (a) & explanation
Pairs 1 and 2 are correct (a). A lack of vitamin C causes scurvy, with bleeding gums, and a lack of vitamin D causes rickets, with soft bones in children. Pair 3 is wrong: night blindness is caused by a lack of vitamin A. Vitamin E deficiency mainly affects nerves and red blood cells.
Other links worth knowing: vitamin B1 and beriberi, B3 and pellagra, B12 and pernicious anaemia, and vitamin K and poor blood clotting. Notice how deficiency diseases tie biology to public policy, such as food fortification and the Vitamin A supplementation programme.
Answer as per the official UPSC answer key.
Consider the following diseases :
- 1. Diphtheria
- 2. Chickenpox
- 3. Smallpox
Which of the above diseases has/have been eradicated in India?
Answer (b) & explanation
Only smallpox has been eradicated (b). India's last case was in 1975, and WHO certified global eradication in 1980, the first disease ever wiped out. Diphtheria still breaks out where vaccine coverage is low, and chickenpox remains common, so options with 1 or 2 fail.
India was declared polio-free in 2014; that is regional elimination, since wild polio still exists in a few countries. Among animal diseases, rinderpest was eradicated in 2011. Notice the language: eradication is worldwide and permanent, elimination is national or regional. The DPT vaccine in the Universal Immunisation Programme protects against diphtheria.
Answer as per the official UPSC answer key.
With reference to two non-conventional energy sources called ‘coalbed methane’ and ‘shale gas’, consider the following statements :
- 1. Coalbed methane is the pure methane gas extracted from coal seams, while shale gas is a mixture of propane and butane only that can be extracted from fine-grained sedimentary rocks.
- 2. In India, abundant coalbed methane sources exist, but so far no shale gas sources have been found.
Which of the statements given above is/are correct?
Answer (d) & explanation
Neither statement is correct (d). Statement 1 is wrong because shale gas is mostly methane, like other natural gas, not just propane and butane; and coalbed methane is mainly methane but not pure. Statement 2 is wrong because India has identified shale basins, including Cambay, Krishna–Godavari, Cauvery and Damodar, and ONGC drilled its first shale well in 2013.
Shale gas is extracted by hydraulic fracturing, which needs huge volumes of water. That has limited its spread in water-stressed India. Notice how the next question on guar gum connects to this same fracking process.
Answer as per the official UPSC answer key.
With reference to Neem tree, consider the following statements :
- 1. Neem oil can be used as a pesticide to control the proliferation of some species of insects and mites.
- 2. Neem seeds are used in the manufacture of biofuels and hospital detergents.
- 3. Neem oil has applications in pharmaceutical industry.
Which of the statements given above is/are correct?
Answer (c) & explanation
Azadirachtin in neem oil repels insects and disrupts their growth, so neem-based products control many insects and mites (1 correct). Neem oil is also used in the pharmaceutical industry, as well as in soaps and toothpastes (3 correct). UPSC's key does not accept statement 2 on biofuels and hospital detergents, so the answer is (c).
India made neem-coated urea compulsory in 2015: the coating slows nitrogen release and stopped urea from being diverted to industry. Neem was also at the centre of a famous biopiracy case, in which a European patent on a neem fungicide was revoked. Notice how traditional knowledge links to IPR.
Answer as per the official UPSC answer key.
Which one of the following is the process involved in photosynthesis?
Answer (b) & explanation
Photosynthesis captures light energy (free energy) and stores it as chemical potential energy in glucose made from carbon dioxide and water, releasing oxygen (b). Option (a) runs the wrong way: releasing stored energy happens in respiration. Options (c) and (d), oxidising food to give CO₂ and water and taking in oxygen, both describe respiration.
Photosynthesis happens in chloroplasts: the light reactions split water and release oxygen, and the dark reactions (Calvin cycle) fix carbon. Notice how the two processes mirror each other and together keep the oxygen and carbon cycles in balance.
Answer as per the official UPSC answer key.
In addition to fingerprint scanning, which of the following can be used in the biometric identification of a person?
- 1. Iris scanning
- 2. Retinal scanning
- 3. Voice recognition
Select the correct answer using the code given below.
Answer (d) & explanation
All three are used (d). Biometrics identifies people by unique physical or behavioural features. Iris scanning reads the coloured ring's patterns (1). Retinal scanning maps blood vessels at the back of the eye (2). Voice recognition analyses the pitch and features of speech (3). Options that leave any of these out are incomplete.
Aadhaar collects fingerprints, iris scans and a face photograph, and face authentication is now widely used too, including in DigiYatra at airports. Notice how biometrics links technology to welfare delivery, while also raising privacy concerns addressed in the Puttaswamy judgment (2017).
Answer as per the official UPSC answer key.
Which of the following statements is/are correct regarding vegetative propagation of plants?
- 1. Vegetative propagation produces clonal population.
- 2. Vegetative propagation helps in eliminating the virus.
- 3. Vegetative propagation can be practiced most of the year.
Select the correct answer using the code given below.
Answer (c) & explanation
Vegetative propagation, by cuttings, grafting, layering or tubers, gives genetically identical clones (1 correct) and can be done for most of the year (3 correct). Statement 2 is wrong: it passes on viruses present in the parent tissue rather than eliminating them. So the answer is (c).
To get virus-free plants, growers use meristem culture, since the growing tip of a plant is often free of virus. Banana, potato and sugarcane are often propagated this way. Notice how clonal crops are uniform but share the same weaknesses, so one disease can wipe out whole fields.
Answer as per the official UPSC answer key.
With reference to the usefulness of the by-products of sugar industry, which of the following statements is/are correct?
- 1. Bagasse can be used as biomass fuel for the generation of energy.
- 2. Molasses can be used as one of the feedstocks for the production of synthetic chemical fertilizers.
- 3. Molasses can be used for the production of ethanol.
Select the correct answer using the codes given below.
Answer (c) & explanation
Statements 1 and 3 are correct (c). Bagasse, the fibrous residue left after crushing cane, is burnt in co-generation plants to produce steam and electricity (1). Molasses, the thick syrup left after sugar crystallises, is fermented into ethanol for blending with petrol and for alcohol (3). Statement 2 is wrong: synthetic fertilisers such as urea are made from natural gas or naphtha, not molasses, though press mud from sugar mills is used as organic manure. So (a), (b) and (d) fail.
Notice the ethanol drive: India reached E20 (20% ethanol in petrol) in 2025, five years ahead of the original target, with sugarcane juice, B-heavy molasses and grains as feedstock. Bagasse is also used to make paper. Trap: organic manure is not synthetic fertiliser.
Answer as per the official UPSC answer key.