Non-conventional energy sources are those energy sources that are:

1. A

   Non-renewable

   (a) Non-renewable. This option claims that non-conventional energy sources cannot be replenished over time. However, non-conventional sources are precisely those that are sustainable and inexhaustible, making this incorrect.

2. B

   Renewable

   (b) Renewable. This option states that non-conventional energy sources are capable of being replenished naturally over a relatively short duration. Solar, wind, and biomass energy fit this definition perfectly, which is why this is the correct official key for the question.

3. C

   Produced from electricity

   (C) Produced from electricity. This option suggests that electricity generation is the defining characteristic of these sources. While some of these sources convert energy into electricity, many do not, meaning this claim is too narrow and conceptually flawed.

4. D

   Produced from heat

   (D) Produced from heat. This option claims that thermal generation defines these energy sources. Although geothermal energy relies on heat, other major non-conventional sources like wind and solar photovoltaic operate on different principles, making this option invalid.

Summary. Official key is (b) Renewable. The question asks for the fundamental characteristic of non-conventional energy sources. Option (a) incorrectly labels them as exhaustible, while options (c) and (d) restrict them to specific secondary forms like electricity or heat. Only option (b) correctly identifies them as renewable and sustainable resources that replace conventional fossil fuels.