Choose the correct group of conventional sources of energy from the following:

Energy sources are grouped as conventional or non-conventional based on how established and widespread their use has been — not simply on whether they are renewable or non-renewable. Conventional sources are the traditional, long-standing sources: fossil fuels (coal, petroleum/oil, natural gas), biomass fuels (firewood, cattle dung cake), and long-established hydel (hydroelectric) and thermal power generation. Non-conventional sources are the comparatively newer alternatives — solar, wind, tidal, geothermal, and biogas energy — and nuclear (atomic) energy is also placed in this non-conventional category.

Oil, natural gas, and firewood are each conventional: oil and natural gas are fossil fuels formed over geological time, and firewood is a traditional biomass fuel that has been used for centuries. Every item in this group is conventional, so this is the matching set.

Checking the other groups shows why each one fails to qualify as a purely conventional set:

• Coal, firewood, biogas — coal and firewood are conventional, but biogas (produced from the anaerobic decomposition of organic waste) is a non-conventional, renewable source, so this group is mixed.

• Hydel power, geothermal energy, tidal energy — hydel (hydroelectric) power is itself a long-established conventional source, but geothermal and tidal energy are non-conventional, so this group mixes categories rather than being purely conventional.

• Nuclear energy, solar energy, hydel power — hydel power is conventional, but nuclear (atomic) energy and solar energy are both non-conventional, so this group is also mixed rather than purely conventional.

Hence, the group made up entirely of conventional sources — oil, natural gas, and firewood — is the correct choice.