An example fossil fuel is

Fossil fuels are energy sources formed over millions of years from the remains of ancient organisms buried under layers of sediment. Heat and pressure gradually transformed this organic matter into carbon-rich fuels. The three main fossil fuels are coal, petroleum (crude oil), and natural gas.

Coal formed from the remains of ancient swamp plants that were buried, compressed, and heated over geological time. It is a combustible sedimentary rock used widely for electricity generation and industrial processes.

The other options are not fossil fuels:

• Clay is a fine-grained sedimentary material composed of clay minerals; it is used in ceramics and construction, not as a fuel.
• Granite is an igneous rock formed from the slow cooling of magma deep within the Earth's crust.
• Sandstone is a sedimentary rock made of cemented sand grains, used as a building material.

The correct answer is coal.