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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What is the common characteristic of the chemical substances generally known as CL-20, HMX and LLM-105, which are sometimes talked about in media?
Answer (b) & explanation
They are explosives used in military weapons (b). CL-20 (hexanitrohexaazaisowurtzitane), HMX (octogen) and LLM-105 are high explosives used in warheads and munitions. CL-20 is among the most powerful non-nuclear explosives. LLM-105 is valued because it is insensitive, so it does not go off easily by accident. Option (a) is wrong: they are not refrigerants. Options (c) and (d) are wrong: they are not fuels for cruise missiles or rockets, which burn steadily rather than detonate.
Notice India's interest: DRDO's High Energy Materials Research Laboratory in Pune works on such compounds. Other common military explosives include RDX and TNT. Some rocket propellants do add HMX for extra energy.
Answer as per the official UPSC answer key.
Consider the following statements :
- I. It is expected that Majorana 1 chip will enable quantum computing.
- II. Majorana 1 chip has been introduced by Amazon Web Services (AWS).
- III. Deep learning is a subset of machine learning.
Which of the statements given above are correct?
Answer (c) & explanation
Statements I and III are correct (c). Statement I is right: Majorana 1 (February 2025) is a quantum chip built on "topological" qubits, expected to make quantum computing more stable. Statement III is right: deep learning, which uses many-layered neural networks, is a subset of machine learning, itself part of AI. Statement II is wrong: Majorana 1 is Microsoft's chip; Amazon's quantum chip is called Ocelot. So options (a), (b) and (d) fail.
Notice Google's quantum chip, Willow (2024). India's National Quantum Mission (2023) aims to build quantum computers with 50–1,000 qubits. A qubit can be 0 and 1 at once (superposition), which gives quantum computers their power.
Big tech firms announced quantum chips within weeks of each other in 2025: Microsoft (Majorana 1), Amazon (Ocelot) and Google (Willow).
Answer as per the official UPSC answer key.
With reference to monoclonal antibodies, often mentioned in news, consider the following statements :
- I. They are man-made proteins.
- II. They stimulate immunological function due to their ability to bind to specific antigens.
- III. They are used in treating viral infections like that of Nipah virus.
Which of the statements given above are correct?
Answer (d) & explanation
All three statements are correct (d). Statement I is right: monoclonal antibodies are man-made proteins, lab-made copies of one antibody. Statement II is right: each binds one specific antigen, marking the target for the immune system or blocking it. Statement III is right: they are used against viral infections; Kerala has used the monoclonal antibody m102.4 against Nipah under compassionate use. So options (a), (b) and (c) fail.
Notice their wide use: in cancer (such as trastuzumab for breast cancer), autoimmune diseases, and COVID-19. They are costly, so making biosimilars at home is a goal of India's biotech policy. The method of making them, hybridoma, won the 1984 Nobel Prize.
Answer as per the official UPSC answer key.
Consider the following statements :
- I. No virus can survive in ocean waters.
- II. No virus can infect bacteria.
- III. No virus can change the cellular transcriptional activity in host cells.
How many of the statements given above are correct?
Answer (d) & explanation
None of the statements is correct (d). Each "no virus" statement is false. Statement I is wrong: seawater is full of viruses, millions in every millilitre; they kill much of the plankton daily and recycle nutrients. Statement II is wrong: bacteriophages are viruses that infect bacteria. Statement III is wrong: viruses routinely take over the host's transcription machinery to make their own proteins. So options (a), (b) and (c) fail.
Notice the medical use of phages: phage therapy is being tried against antibiotic-resistant bacteria, a growing threat called AMR. Viruses are not truly alive outside a host cell. Some viruses, like HPV, can even cause cancer by changing how host genes are switched on.
Answer as per the official UPSC answer key.
Consider the following statements :
Statement I : Some rare earth elements are used in the manufacture of flat television screens and computer monitors.
Statement II : Some rare earth elements have phosphorescent properties.
Which one of the following is correct in respect of the above statements?
Answer (a) & explanation
Both are correct and Statement II explains Statement I (a). Statement I is right: some rare earth elements are used in flat TV screens and monitors. Statement II is right and is the reason: rare earths such as europium, terbium and yttrium are used as phosphors, the materials that glow red, green and blue in screens and LED lighting. Their light-giving property is why screens use them. So options (b), (c) and (d) fail.
Notice their other uses: neodymium magnets in EV motors and wind turbines. China controls most rare earth processing, and its export curbs in 2025 hit carmakers worldwide. India's IREL mines rare earths from beach sands in Kerala and Odisha, and a 2025 scheme backs rare-earth magnet making.
Answer as per the official UPSC answer key.
Which one of the following words/phrases is most appropriately used to denote “an interoperable network of 3D virtual worlds that can be accessed simultaneously by millions of users, who can exert property rights over virtual items” ?
Answer (c) & explanation
This describes the metaverse (c). The metaverse is a persistent, shared, interoperable network of 3D virtual worlds that millions of users can enter at the same time, through VR/AR headsets or screens. Users can own virtual land and items, often recorded on blockchains as NFTs. Option (a) is wrong: big data analytics studies huge data sets for patterns. Option (b) is wrong: cryptography is the science of secure codes. Option (d) is wrong: "virtual matrix" is not a standard term.
Notice that the word comes from Neal Stephenson's 1992 novel Snow Crash. Facebook renamed itself Meta in 2021 to bet on it. The question's wording follows a widely cited definition by investor Matthew Ball.
Answer as per the official UPSC answer key.
Consider the following :
- 1. Battery storage
- 2. Biomass generators
- 3. Fuel cells
- 4. Rooftop solar photovoltaic units
How many of the above are considered “Distributed Energy Resources” ?
Answer (d) & explanation
All four are Distributed Energy Resources (d). Distributed Energy Resources (DERs) are small units that generate or store power close to where it is used, often behind the consumer's meter, rather than at a big central plant. Battery storage (item 1) stores power for later. Biomass generators (item 2) make power from crop waste locally. Fuel cells (item 3) make electricity from hydrogen on site. Rooftop solar (item 4) is the most common DER. Micro-wind turbines and EV chargers that feed back power also count.
Notice the benefits: less power lost in long lines, more resilience during outages, and more renewables. India's PM Surya Ghar scheme (2024) aims at rooftop solar for one crore homes. Grouping many DERs together creates a virtual power plant.
Answer as per the official UPSC answer key.
With reference to radioisotope thermoelectric generators (RTGs), consider the following statements :
- 1. RTGs are miniature fission reactors.
- 2. RTGs are used for powering the onboard systems of spacecrafts.
- 3. RTGs can use Plutonium-238, which is a by-product of weapons development.
Which of the statements given above are correct ?
Answer (b) & explanation
Statements 2 and 3 are correct (b). Statement 2 is right: RTGs power the onboard systems of spacecraft, especially deep-space probes such as Voyager and New Horizons, and Mars rovers like Curiosity and Perseverance, where sunlight is too weak or dust blocks solar panels. Statement 3 is right: they use plutonium-238, made in reactors linked to weapons programmes. Statement 1 is wrong: an RTG is not a reactor and has no chain reaction. It turns the heat of natural radioactive decay into electricity through thermocouples.
Notice that India's Chandrayaan-3 lander carried small radioisotope heater units from BARC to test survival through the cold lunar night. Pu-238 has a half-life of about 88 years, giving decades of steady power.
Answer as per the official UPSC answer key.
Consider the following statements :
Statement-I : Giant stars live much longer than dwarf stars.
Statement-II : Compared to dwarf stars, giant stars have a greater rate of nuclear reactions.
Which one of the following is correct in respect of the above statements ?
Answer (d) & explanation
Statement I is wrong but Statement II is correct (d). Statement II is right: giant, massive stars have much hotter, denser cores, so their nuclear reactions run far faster than in small dwarf stars. Statement I is wrong: because they burn fuel so fast, giant stars live shorter lives. A very massive star may last only a few million years, while a small red dwarf can shine for trillions of years. More fuel does not mean a longer life: the faster burn rate wins.
Notice the endings: massive stars die in a supernova, leaving a neutron star or black hole. Sun-like stars swell into red giants and end as white dwarfs. The Chandrasekhar limit (about 1.4 solar masses) sets the maximum mass of a white dwarf.
Answer as per the official UPSC answer key.
Which one of the following is synthesised in human body that dilates blood vessels and increases blood flow ?
Answer (a) & explanation
Nitric oxide dilates blood vessels (a). The inner lining of blood vessels makes nitric oxide (NO) from the amino acid arginine. NO relaxes the smooth muscle in vessel walls, which widens them and increases blood flow, lowering blood pressure. Nitroglycerine for angina and drugs like sildenafil work through this pathway. Option (b) is wrong: nitrous oxide is laughing gas, used in anaesthesia and a greenhouse gas. Option (c) is wrong: nitrogen dioxide is a toxic air pollutant from vehicles. Option (d) is wrong: nitrogen pentoxide is an industrial chemical, not made by the body.
Notice that the discovery of NO as a body signal won the 1998 Nobel Prize in Medicine. Beetroot and leafy greens supply nitrates that the body turns into NO.
Answer as per the official UPSC answer key.
Consider the following activities :
- 1. Identification of narcotics on passengers at airports or in aircraft
- 2. Monitoring of precipitation
- 3. Tracking the migration of animals
In how many of the above activities can the radars be used ?
Answer (b) & explanation
Radars can be used in two of the three (b). Item 2 is right: Doppler weather radars track rain, hail and storms, and help forecast cyclones; India's IMD runs a growing radar network, expanded under Mission Mausam (2024). Item 3 is right: the same radars pick up flocks of birds, bats and swarms of insects such as locusts on migration, and scientists use this to study them. Item 1 is wrong under UPSC's key: narcotics on passengers are found with X-ray and body scanners, chemical trace detectors and sniffer dogs, not radar.
Notice that radar stands for Radio Detection and Ranging. It bounces radio waves off objects, and the Doppler shift in the echo shows speed and direction.
Answer as per the official UPSC answer key.
In which of the following are hydrogels used ?
- 1. Controlled drug delivery in patients
- 2. Mobile air-conditioning systems
- 3. Preparation of industrial lubricants
Select the correct answer using the code given below :
Answer (a) & explanation
Hydrogels are used in controlled drug delivery (a). Hydrogels are networks of polymer chains that can soak up and hold large amounts of water while keeping their shape, like jelly. Item 1 is right: they are used for controlled drug release, letting medicine leak out slowly in the body. They are also used in soft contact lenses, wound dressings, nappies and to hold moisture in farm soil. Items 2 and 3 are not accepted by UPSC's key: hydrogels are not used in air-conditioning systems or in making industrial lubricants.
Notice the farm use: hydrogel granules mixed in soil soak up rain and release it to roots slowly, helping crops survive dry spells. India's Pusa Hydrogel, from IARI, is one such product.
Answer as per the official UPSC answer key.
Recently, the term “pumped-storage hydropower” is actually and appropriately discussed in the context of which one of the following ?
Answer (c) & explanation
Pumped-storage hydropower is discussed for long-duration energy storage (c). Pumped storage uses spare power, often cheap solar power at midday, to pump water up to a higher reservoir. At peak demand, the water is released through turbines to make electricity again. It works like a giant battery for long-duration storage, and helps fit variable renewables like solar and wind into the grid. Options (a), (b) and (d), about terrace farming, lift irrigation and rainwater harvesting, have nothing to do with it.
Notice India's push: the Power Ministry issued guidelines in 2023 to promote pumped storage projects. India has large potential, over 100 GW. Pumped storage makes up most of the world's grid storage today.
Answer as per the official UPSC answer key.
Which one of the following countries has its own Satellite Navigation System?
Answer (d) & explanation
Japan has its own satellite navigation system (d). Japan's Quasi-Zenith Satellite System (QZSS), called Michibiki, is a regional system that boosts GPS accuracy over Japan and the Asia–Oceania region. Australia (a), Canada (b) and Israel (c) rely on others' systems. There are four global systems: GPS (USA), GLONASS (Russia), Galileo (European Union) and BeiDou (China). There are two regional systems: Japan's QZSS and India's NavIC.
Notice India's NavIC (IRNSS) covers India and about 1,500 km around it. Some of its satellites lost their atomic clocks, and ISRO is launching the new NVS series, starting with NVS-01 in 2023. Phones now carry chips that use NavIC.
Answer as per the official UPSC answer key.
‘Wolbachia method’ is sometimes talked about with reference to which one of the following?
Answer (a) & explanation
The Wolbachia method is used to control viral diseases spread by mosquitoes (a). Wolbachia is a common bacterium found in many insects. When it is put into Aedes aegypti mosquitoes, it stops dengue, Zika and chikungunya viruses from multiplying inside them. Infected females pass it to their young, so it spreads through the wild mosquito population over time. Options (b), (c) and (d) are about crop residues, bioplastics and biochar, which have nothing to do with it.
Notice the results: the World Mosquito Program has cut dengue sharply in trial cities such as Yogyakarta in Indonesia. India's ICMR developed its own Wolbachia strains of Aedes mosquitoes in 2024. Another approach releases sterile male mosquitoes.
Answer as per the official UPSC answer key.
‘Microsatellite DNA’ is used in the case of which one of the following?
Answer (a) & explanation
Microsatellite DNA is used to study evolutionary relationships (a). Microsatellites are short DNA sequences, two to six letters long, repeated many times in a row, such as CACACA. The number of repeats varies a lot between individuals and populations. That makes them useful for DNA fingerprinting, paternity tests, wildlife forensics, population genetics and studying evolutionary relationships among species. Option (b) is about stem cells, option (c) about tissue culture and option (d) about clinical trials. "Satellite" here has nothing to do with space.
Notice India's use: the Wildlife Institute of India and CCMB's LaCONES lab use such markers to identify tiger individuals from scat and catch poachers. They also help breeding programmes avoid inbreeding.
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. Carbon fibres are used in the manufacture of components used in automobiles and aircrafts.
- 2. Carbon fibres once used cannot be recycled.
Which of the statements given above is/are correct?
Answer (a) & explanation
Only statement 1 is correct (a). Statement 1 is right: carbon fibre composites are very strong and light, so they are used in aircraft such as the Boeing 787 and Airbus A350, in racing and electric cars, wind turbine blades and sports gear like rackets and bicycles. Lighter vehicles burn less fuel. Statement 2 is wrong: carbon fibres can be recycled. Processes such as pyrolysis, which burns away the resin, and solvolysis, which dissolves it, recover the fibres, though these are costly and the fibres come out shorter.
Notice India's push: carbon fibre is a strategic material for missiles and space vehicles, and India still imports most of it. ISRO and DRDO have built their own, and private firms are setting up plants.
Answer as per the official UPSC answer key.
Consider the following actions :
- 1. Detection of car crash/collision which results in the deployment of airbags almost instantaneously
- 2. Detection of accidental free fall of a laptop towards the ground which results in the immediate turning off of the hard drive
- 3. Detection of the tilt of the smartphone which results in the rotation of display between portrait and landscape mode
In how many of the above actions is the function of accelerometer required?
Answer (c) & explanation
An accelerometer is needed in all three (c). An accelerometer, usually a tiny MEMS chip, senses changes in speed and the direction of gravity. Action 1 needs it: in a crash, it detects the sudden deceleration and fires the airbags within milliseconds. Action 2 needs it: when a laptop is in free fall, the sensor feels near-zero gravity and parks the hard drive heads to avoid damage. Action 3 needs it: it measures the tilt of a phone against gravity to switch the screen between portrait and landscape.
Notice its partners: a gyroscope measures rotation, and a magnetometer works like a compass. Together they power step counters, games and navigation when GPS drops. Smartwatches also use them for fall detection.
Answer as per the official UPSC answer key.
Consider the following pairs :
| Objects in space | Description
| 1. Cepheids | Giant clouds of dust and gas in space
| 2. Nebulae | Stars which brighten and dim periodically
| 3. Pulsars | Neutron stars that are formed when massive stars run out of fuel and collapse
How many of the above pairs are correctly matched?
Answer (a) & explanation
Only one pair is correct (a). Pair 3 is right: pulsars are fast-spinning neutron stars, left when a massive star runs out of fuel and collapses in a supernova; they send out beams of radio waves like a lighthouse. Pairs 1 and 2 are swapped. Cepheids are pulsating stars that brighten and dim regularly; their period shows their true brightness, so they are used to measure cosmic distances. Nebulae are giant clouds of dust and gas, where new stars are born.
Notice that Henrietta Leavitt found the Cepheid period-brightness rule, and Edwin Hubble used it to show the universe is expanding. India's GMRT near Pune helps study pulsars, which act as precise cosmic clocks.
Answer as per the official UPSC answer key.
With reference to “Software as a Service (SaaS)”, consider the following statements :
- 1. SaaS buyers can customise the user interface and can change data fields.
- 2. SaaS users can access their data through their mobile devices.
- 3. Outlook, Hotmail and Yahoo! Mail are forms of SaaS.
Which of the statements given above are correct ?
Answer (d) & explanation
All three statements are correct (d). In Software as a Service, the provider hosts the software in the cloud, and users reach it through a browser or app. Statement 1 is right: buyers can usually configure the user interface and change data fields to suit their needs. Statement 2 is right: users can reach their data from any device, including mobiles, over the internet. Statement 3 is right: webmail services such as Outlook, Hotmail and Yahoo! Mail are everyday examples of SaaS.
Notice the three cloud layers: SaaS (ready software, such as Gmail), PaaS (Platform as a Service, a platform for developers) and IaaS (Infrastructure as a Service, raw servers and storage). India's government cloud initiative is MeghRaj.
Answer as per the official UPSC answer key.