Which one of the following non-conventional energy sources can be exploited…

Concept: How economically a non-conventional energy source can be exploited is decided by three things taken together — how widely the resource itself occurs, how much capital and technology one conversion unit needs, and whether the source can be used at small scale without a grid. A source rates highest when its resource is available almost everywhere, its conversion device needs no fuel, and it stays viable at every size from a single household appliance up to a power station.

Application: Sunlight reaches practically the whole of India, which receives roughly 4–7 kWh/m2 per day over about 250–300 clear days a year. The amount does vary with region, season and cloud cover, but unlike the other three sources solar energy is not confined to a few special sites. The conversion devices also span the entire cost range: a solar cooker, a flat-plate water heater or a solar dryer needs no fuel, no grid connection and very little capital, while photovoltaic modules and concentrating collectors cover the utility-scale end. Because the resource is available almost everywhere and even the cheapest devices are already useful, solar energy can be tapped economically over most of the country.

Cross-check — why the other three sources are harder to exploit as widely and as cheaply:

• Wind: the power a rotor can extract varies with the cube of the wind speed, so generation is worthwhile only in corridors with mean speeds above roughly 5 m/s — the coastal and hill belts of Tamil Nadu, Gujarat, Maharashtra, Karnataka and Rajasthan. Small stand-alone wind machines do exist, but outside those corridors their output is too low to repay the investment, and grid-scale wind needs turbines plus evacuation infrastructure.

• Geo-thermal: commercial generation needs a high-enthalpy reservoir, typically hotter than about 150 °C, formed where recent volcanic or tectonic activity brings heat close to the surface. India’s known fields — Puga in Ladakh, Manikaran in Himachal Pradesh and Tattapani in Chhattisgarh — are low-to-medium enthalpy, and no commercial geothermal power plant operates in the country.

• Ocean Thermal Energy Conversion (OTEC): it runs a heat engine on the roughly 20 °C difference between warm surface water and cold water pumped up from about 1000 m depth. That small temperature difference limits net efficiency to only a few per cent, and the cold-water pipes are enormous and costly, so OTEC is still at the pilot and demonstration stage worldwide.

Result: the criterion this question applies is how widely and how cheaply a source can actually be tapped — availability over most of the country, freedom from fuel, and viability at every scale — and on that criterion the answer is solar energy. A note on scope: judged instead purely by the utility-scale generation tariff of the exam’s own period, wind power was then cheaper per unit than solar photovoltaic, so a student who reasons from tariff alone will reach wind. The keyed answer follows the availability-and-scale reading rather than the tariff reading.