A non-renewable source of energy is–

Petroleum forms from buried organic matter over millions of years under heat and pressure, so the stock we can extract is finite and its replenishment is effectively zero on human timescales. That makes it non-renewable. The core concept is that a renewable source is replenished at a rate comparable to its consumption, and petroleum fails that test.

• Solar energy (A): Sunlight is renewable. The sun will supply it for billions of years for practical purposes.
• Wind energy (C): Driven by uneven atmospheric heating and Earth's rotation, so it is continually regenerated.
• Biogas (D): Produced by anaerobic digestion of cattle dung, crop residue and organic waste, which are continuously replenished, so it is renewable.

Coal, petroleum and natural gas are the classic fossil fuels in the Energy Resources part of Geography. The Mains angle is energy security, since India imports roughly 85 per cent of its crude oil requirement in recent years.

Trap: Aspirants sometimes pick biogas because it is derived from waste and burns like a fuel. The test is whether the feedstock regenerates, not whether it burns.