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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Which of the following leaf modifications occurs/occur in desert areas to inhibit water loss?
- 1. Hard and waxy leaves
- 2. Tiny leaves or no leaves
- 3. Thorns instead of leaves
Select the correct answer using the codes given below.
Answer (d) & explanation
All three are correct (d). Desert plants (xerophytes) cut transpiration in every way. Hard, waxy leaves with a thick cuticle reduce evaporation (1). Tiny leaves or no leaves reduce the surface losing water (2). Thorns or spines instead of leaves, as in cactus, reduce water loss further, while the green stem does photosynthesis (3). So options (a), (b) and (c) leave out real adaptations.
Notice the other adaptations: sunken stomata, stomata that open at night (CAM photosynthesis in cacti and pineapple), deep tap roots, and fleshy succulent stems that store water. India's Thar has khejri, which has deep roots, and ker with tiny leaves. Trap: all three are textbook xerophyte features.
Answer as per the official UPSC answer key.
Improper handling and storage of cereal grains and oilseeds result in the production of toxins known as aflatoxins which are not generally destroyed by normal cooking process. Aflatoxins are produced by
Answer (c) & explanation
Moulds is correct (c). Aflatoxins are produced by the moulds (fungi) Aspergillus flavus and A. parasiticus, which grow on damp or poorly stored groundnuts, maize, cottonseed, spices and other grains. They are potent liver toxins and among the strongest natural carcinogens, and normal cooking does not destroy them. (a) is wrong: bacteria cause other food poisoning, such as botulinum toxin. (b) and (d) are wrong: protozoa and viruses do not make aflatoxins.
Notice the safety rules: FSSAI sets limits on aflatoxin in food, and aflatoxin M1 appears in milk when cattle eat contaminated feed. Good drying to low moisture and proper storage prevent it. Aflatoxin rejections have hit Indian groundnut and spice exports to the EU. Trap: toxins in food are often blamed on bacteria.
Answer as per the official UPSC answer key.
The known forces of nature can be divided into four classes, viz., gravity, electromagnetism, weak nuclear force and strong nuclear force. With reference to them, which one of the following statements is *not* correct?
Answer (a) & explanation
Gravity is the strongest is the statement not correct (a). Gravity is the weakest of the four forces; it dominates only at large scales because masses always attract and add up, while electric charges cancel. The strong nuclear force is the strongest. (b) is correct: electromagnetism acts on charged particles. (c) is correct: the weak nuclear force causes beta decay, a form of radioactivity. (d) is correct: the strong force holds protons and neutrons in the nucleus despite the protons' repulsion.
Notice the ranking: strong (1) > electromagnetic (about 1/137) > weak > gravity (about 10⁻³⁸). Electromagnetism and the weak force are unified as the electroweak force (Nobel 1979). Gravity is explained by general relativity, not yet joined with quantum theory. Trap: gravity feels powerful in daily life but is the weakest.
Answer as per the official UPSC answer key.
Consider the following organisms :
- 1. Agaricus
- 2. Nostoc
- 3. Spirogyra
Which of the above is/are used as biofertilizer/biofertilizers?
Answer (b) & explanation
Nostoc only is correct (b). Nostoc is a blue-green alga (cyanobacterium) that fixes atmospheric nitrogen in special cells called heterocysts, so it is used as a biofertiliser in paddy fields, along with Anabaena (2). Item 1 is wrong: Agaricus is an edible mushroom, a fungus that does not fix nitrogen. Item 3 is wrong: Spirogyra is a green alga that does not fix nitrogen. So (a), (c) and (d) fail.
Notice the other biofertilisers: Rhizobium in legume root nodules, free-living Azotobacter and Azospirillum, the Azolla–Anabaena partnership in rice fields, and mycorrhizal fungi that help absorb phosphorus. Biofertilisers are promoted under the National Mission on Natural Farming. Trap: all three are 'lower' organisms, but only one fixes nitrogen.
Answer as per the official UPSC answer key.
The efforts to detect the existence of Higgs boson particle have become frequent news in the recent past. What is/are the importance/importances of discovering this particle?
- 1. It will enable us to understand as to why elementary particles have mass.
- 2. It will enable us in the near future to develop the technology of transferring matter from one point to another without traversing the physical space between them.
- 3. It will enable us to create better fuels for nuclear fission.
Select the correct answer using the codes given below.
Answer (a) & explanation
Statement 1 only is correct (a). The Higgs boson, discovered at CERN's Large Hadron Collider in July 2012, confirmed the Higgs field, which explains why elementary particles have mass (1). Statement 2 is wrong: it has no link to teleporting matter. Statement 3 is wrong: it does not help make nuclear fuels. So (b), (c) and (d) fail.
Notice the Indian link: the particle is a boson, a class named after Satyendra Nath Bose. Peter Higgs and François Englert won the 2013 Nobel Prize in Physics. India became an associate member of CERN in 2017. The popular name 'God particle' comes from a book title, not from physics. Trap: dramatic applications are attached to basic science.
Answer as per the official UPSC answer key.
Which of the following diseases can be transmitted from one person to another through tattooing?
- 1. Chikungunya
- 2. Hepatitis B
- 3. HIV-AIDS
Select the correct answer using the codes given below.
Answer (b) & explanation
Hepatitis B and HIV are correct (b). Tattoo needles shared or reused without sterilising can carry blood-borne viruses: Hepatitis B (2), Hepatitis C, and HIV (3). Item 1 is wrong: chikungunya spreads through the bite of infected Aedes mosquitoes (aegypti and albopictus), not through blood contact in tattooing. So (a), (c) and (d) fail.
Notice the prevention: single-use needles, sterilised equipment and fresh ink pots. India's blood banks screen donors for HBV, HCV and HIV and often defer people with recent tattoos for 6–12 months. Hepatitis B has a vaccine in India's Universal Immunisation Programme; HIV and Hepatitis C do not. Trap: chikungunya is a viral disease, but its route is the mosquito.
Answer as per the official UPSC answer key.
Mycorrhizal biotechnology has been used in rehabilitating degraded sites because mycorrhiza enables the plants to
- 1. resist drought and increase absorptive area
- 2. tolerate extremes of pH
- 3. resist disease infestation
Select the correct answer using the codes given below.
Answer (d) & explanation
All three are correct (d). Mycorrhiza is a partnership between a fungus and plant roots. The fungal threads extend the absorbing area of roots far into the soil, helping the plant take up water and resist drought (1). They help plants tolerate extremes of pH, salinity and heavy metals in poor soils (2). They also protect roots against soil pathogens by forming a barrier and competing with them (3). This is why mycorrhizal inoculants are used to rehabilitate degraded sites such as mine spoils. So (a), (b) and (c) leave out real benefits.
Notice the main benefit: mycorrhiza greatly improves phosphorus uptake. About 80% of land plants form such partnerships. TERI in India developed mycorrhizal biofertilisers for wasteland reclamation. Trap: all three effects are documented.
Answer as per the official UPSC answer key.
Consider the following minerals :
- 1. Calcium
- 2. Iron
- 3. Sodium
Which of the minerals given above is/are required by human body for the contraction of muscles?
Answer (d) & explanation
All three per UPSC's key (d), but iron's place is doubtful. Sodium ions rush into nerve and muscle cells to create the action potential that triggers contraction (3). Calcium released inside the muscle cell binds troponin and lets actin and myosin slide (1). Iron is not directly part of the contraction mechanism; it sits in myoglobin and haemoglobin, supplying the oxygen muscles need to keep working, so many argue for (c). UPSC counted iron as required. (a) and (b) fail on any reading.
Notice the others: potassium restores the resting state, and magnesium helps muscles relax; low calcium causes tetany and cramps. Anaemia from iron deficiency, common in India, causes muscle fatigue. Trap: 'required for contraction' can be read narrowly or broadly.
Answer as per the official UPSC answer key.
Fruits stored in a cold chamber exhibit longer storage life because
Answer (c) & explanation
The rate of respiration is decreased is correct (c). Harvested fruit is still alive and keeps respiring, burning its stored sugars, losing water and ripening further. Low temperature slows respiration and enzyme action, and also slows the growth of microbes that cause rot, so the fruit lasts longer. (a) is wrong: cold chambers are dark, but blocking sunlight is not the main reason. (b) is wrong: higher CO₂ helps only in special controlled-atmosphere storage, not in a simple cold chamber. (d) is wrong: humidity is kept moderate to prevent shrivelling, but it is not the main reason.
Notice the related methods: ethylene scrubbing slows ripening, and irradiation extends shelf life of onions and potatoes. India loses a large share of fruits and vegetables after harvest; the Integrated Cold Chain scheme under PM Kisan SAMPADA tackles this. Trap: cold slows life processes; that is the core idea.
Answer as per the official UPSC answer key.
Ball bearings are used in bicycles, cars, etc., because
Answer (c) & explanation
The effective area of contact is reduced is correct (c). Ball bearings replace sliding with rolling. The balls touch the races only at tiny points, so the effective area of contact is reduced, and rolling friction is far less than sliding friction. Less friction means less heat, wear and energy loss. (a) and (b) are wrong: more contact area would increase friction. (d) fails because (c) is correct.
Notice the related ideas: lubricants such as oil and grease add a thin film to cut friction further, and roller bearings handle heavier loads. Friction is not always bad: tyres need grip, and treads increase it on roads. The invention of the wheel was the first use of rolling over sliding. Trap: 'actual' versus 'effective' area is the subtle distinction.
Answer as per the official UPSC answer key.
Consider the following phenomena :
- 1. Size of the sun at dusk
- 2. Colour of the sun at dawn
- 3. Moon being visible at dawn
- 4. Twinkle of stars in the sky
- 5. Polestar being visible in the sky
Which of the above are optical illusions?
Answer (c) & explanation
Items 1, 2 and 4 are correct (c). The sun looks larger near the horizon at dusk, a perception effect called the moon illusion, which also applies to the sun (1). Its reddish colour at dawn comes from scattering of shorter blue wavelengths as light travels through more air (2). The twinkling of stars comes from refraction by moving layers of air of different density (4). Items 3 and 5 are wrong: the moon at dawn and the pole star are really there, so they are not illusions. So (a), (b) and (d) fail.
Notice the related effects: atmospheric refraction lets us see the sun about two minutes before it actually rises and after it sets. Planets do not twinkle much because they appear as discs, not points. Trap: 'colour' is a real scattering effect, but UPSC groups it with appearances.
Answer as per the official UPSC answer key.
Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this?
- 1. Dispersion
- 2. Refraction
- 3. Internal reflection
Select the correct answer using the codes given below.
Answer (d) & explanation
All three are correct (d). Sunlight is refracted, or bent, as it enters a raindrop (2), and split into its colours because each wavelength bends differently: dispersion (1). The light is then reflected internally at the back of the drop (3) and refracted again as it leaves. Together, these make a rainbow at about 42° from the point opposite the sun. So options (a), (b) and (c) leave out real processes.
Notice the related facts: a rainbow is seen only with the sun behind you; a secondary rainbow, from two internal reflections, has reversed colours and lies outside the main bow. Red is on the outside of the primary rainbow. Newton used a prism to show dispersion. Trap: internal reflection is often forgotten.
Answer as per the official UPSC answer key.
Many transplanted seedlings do not grow because
Answer (c) & explanation
Most of the root hairs are lost is correct (c). Root hairs, thin outgrowths near the root tips, absorb most of a plant's water and minerals. They are delicate and are torn off when a seedling is uprooted, so the transplanted plant cannot take up enough water and wilts, sometimes dying. (a) is wrong: the new soil usually has adequate minerals. (b) is wrong: root hairs do not 'grip too hard'. (d) is wrong: leaf damage can happen but is not the main reason.
Notice the practice: gardeners transplant with a ball of soil around the roots, water well, shade the seedlings, and transplant in the evening to cut transpiration. Paddy is transplanted from nurseries, while Direct Seeded Rice avoids transplanting altogether and saves water. Trap: the problem is at the roots, not the soil.
Answer as per the official UPSC answer key.
Recombinant DNA technology (Genetic Engineering) allows genes to be transferred
- 1. across different species of plants
- 2. from animals to plants
- 3. from microorganisms to higher organisms
Select the correct answer using the codes given below.
Answer (d) & explanation
All three are correct (d). Because the genetic code is universal, a gene from one organism can work in another. Recombinant DNA technology therefore allows transfer across plant species (1), from animals to plants, as in trials with a fish antifreeze gene in tomato (2), and from microbes to higher organisms, as with the Bt gene in cotton and the human insulin gene in bacteria in reverse (3). So options (a), (b) and (c) leave out real possibilities.
Notice the tools: restriction enzymes cut DNA, ligase joins it, and plasmids or Agrobacterium carry it. Newer CRISPR-Cas9 editing (Nobel 2020) changes a plant's own genes; India exempted such SDN-1 and SDN-2 edits from GM rules in 2022. Trap: the universality of the code makes all three transfers possible.
Answer as per the official UPSC answer key.
Which of the following statements is/are correct?
- 1. Viruses lack enzymes necessary for the generation of energy.
- 2. Viruses can be cultured in any synthetic medium.
- 3. Viruses are transmitted from one organism to another by biological vectors only.
Select the correct answer using the codes given below.
Answer (a) & explanation
Statement 1 only is correct (a). Viruses have no energy-making enzymes or metabolism of their own; they use the host cell's machinery to make copies (1). Statement 2 is wrong: viruses grow only in living cells, such as cell cultures or embryonated eggs, not in a synthetic, cell-free medium. Statement 3 is wrong: viruses spread by many routes: air (influenza, COVID-19), water and food (hepatitis A), contact, blood (HIV) and also vectors (dengue). So (b), (c) and (d) fail.
Notice the debate: viruses are at the edge of life, active only inside cells. Bacteriophages infect bacteria and are studied as alternatives to antibiotics. India's NIV, Pune is the main virology institute. Trap: 'only' makes statement 3 too narrow.
Answer as per the official UPSC answer key.
With reference to ‘stem cells’, frequently in the news, which of the following statements is/are correct?
- 1. Stem cells can be derived from mammals only.
- 2. Stem cells can be used for screening new drugs.
- 3. Stem cells can be used for medical therapies.
Select the correct answer using the codes given below :
Answer (b) & explanation
Statements 2 and 3 are correct (b). Stem cells are unspecialised cells that can divide and turn into other cell types. They are used to grow tissues in the lab to screen new drugs for safety and effect (2), and in therapies such as bone-marrow transplants for leukaemia (3). Statement 1 is wrong: stem cells exist in all multicellular organisms, including plants (meristems), not only mammals. So (a), (c) and (d) fail.
Notice the types: embryonic (pluripotent), adult (multipotent) and induced pluripotent stem cells (iPSCs), for which Shinya Yamanaka shared the 2012 Nobel Prize. India's ICMR guidelines (2017) permit only approved stem-cell therapies, mainly bone-marrow transplants. Trap: 'mammals only' is too narrow.
Answer as per the official UPSC answer key.
A team of scientists at Brookhaven National Laboratory including those from India created the heaviest anti-matter (anti-helium nucleus). What is/are the implication/implications of the creation of anti-matter?
- 1. It will make mineral prospecting and oil exploration easier and cheaper.
- 2. It will help probe the possibility of the existence of stars and galaxies made of anti-matter.
- 3. It will help understand the evolution of the universe.
Select the correct answer using the codes given below :
Answer (b) & explanation
Statements 2 and 3 are correct (b). The anti-helium-4 nucleus, the heaviest antimatter nucleus then made, was detected at the RHIC collider at Brookhaven (2011), with Indian scientists in the STAR team. Such work helps search for antimatter stars and galaxies (2) and understand why the early universe ended up with more matter than antimatter (3). Statement 1 is wrong: antimatter has no use in mineral or oil exploration; it is made in tiny amounts and vanishes on contact with matter. So (a), (c) and (d) fail.
Notice the related tools: the Alpha Magnetic Spectrometer on the ISS searches for cosmic antimatter. Positrons (anti-electrons) are used in medical PET scans. Trap: practical-sounding uses are attached to basic physics.
Answer as per the official UPSC answer key.
Which of the following is/are cited by the scientists as evidence/evidences for the continued expansion of universe?
- 1. Detection of microwaves in space
- 2. Observation of redshift phenomenon in space
- 3. Movement of asteroids in space
- 4. Occurrence of supernova explosions in space
Select the correct answer using the codes given below :
Answer (a) & explanation
Microwaves and redshift are correct (a). Light from distant galaxies is red-shifted: its wavelength is stretched because the galaxies are moving away, faster the farther they are (Hubble's law) (2). The cosmic microwave background radiation, found in 1965, is the cooled afterglow of the Big Bang, spread evenly across the sky (1). Statement 3 is wrong: asteroids move within our solar system and say nothing about expansion. Statement 4 is wrong: supernovae alone are not evidence, though Type Ia supernovae used as distance markers showed in 1998 that expansion is accelerating. So (b), (c) and (d) fail.
Notice the puzzle: this acceleration is blamed on dark energy, and the 'Hubble tension' is a mismatch in measured expansion rates. Trap: (4) hides a partial truth.
Answer as per the official UPSC answer key.
To meet its rapidly growing energy demand, some opine that India should pursue research and development on thorium as the future fuel of nuclear energy. In this context, what advantage does thorium hold over uranium?
- 1. Thorium is far more abundant in nature than uranium.
- 2. On the basis of per unit mass of mined mineral, thorium can generate more energy compared to natural uranium.
- 3. Thorium produces less harmful waste compared to uranium.
Which of the statements given above is/are correct?
Answer (d) & explanation
All three are correct per UPSC's key (d). Thorium is about three to four times more abundant than uranium in the Earth's crust, and India holds about a quarter of world reserves in monazite sands of Kerala, Tamil Nadu and Odisha (1). Nearly all mined thorium-232 can be bred into fissile uranium-233, unlike the 0.7% U-235 in natural uranium, so it can yield more energy per tonne mined (2). The thorium cycle produces far fewer long-lived transuranic wastes such as plutonium (3). So (a), (b) and (c) leave out real advantages.
Notice India's plan: thorium is stage three of Bhabha's three-stage programme; the Prototype Fast Breeder Reactor at Kalpakkam began core loading in 2024. Thorium is not fissile itself, so it needs a starter fuel. Trap: 'more energy' holds only via breeding.
Answer as per the official UPSC answer key.
Which one of the following sets of elements was primarily responsible for the origin of life on the Earth?
Answer (b) & explanation
Carbon, hydrogen and nitrogen is correct (b). Life's building blocks, amino acids, sugars, fats and nucleic acids, are made mainly of carbon, hydrogen, nitrogen and oxygen, with phosphorus and sulphur. The Miller–Urey experiment (1953) made amino acids by passing sparks through methane, ammonia, hydrogen and water, mimicking the early Earth. (a) and (d) are wrong: sodium and potassium are minerals important for cells but not the core of organic molecules. (c) is wrong: calcium is not primary, and the early atmosphere had little free oxygen.
Notice the related theories: Oparin–Haldane's 'primordial soup', the RNA world, and life near deep-sea hydrothermal vents. Oxygen built up later through cyanobacteria in the Great Oxidation Event (about 2.4 billion years ago). Trap: oxygen is essential now but was not primary then.
Answer as per the official UPSC answer key.