Fuel formed under the earth's surface by the decomposition of organic matter is called

Fuel formed underground by the slow decomposition of buried organic matter is called fossil fuel.

Inorganic fuel is not the term for such organically derived deposits.

Biogas is made fresh from decaying waste, not formed underground over ages.

Since fossil fuel fits exactly, none-of-these does not apply.

Coal, petroleum and natural gas are the main fossil fuels.

They took millions of years to form under great heat and pressure.