Fossil fuels are formed from living beings that die, such as trees or animals. After many years of being covered by several layers of soil and being subjected to high pressure and temperature, they become fossil fuel. So the answer could be decayed remains of plants or animals.
For these fuels to generate the energy they must undergo a combustion reaction using oxygen as the oxidizing agent. Another way of saying it is that the fuel burns to form energy. So this could be the answer.
Solid fossil fuels are formed from plant or animal remains. Produces energy when the fuel burns.