What is the ultimate source of energy in fossil fuel?
Fossil fuel: Fossil fuel is the source of energy found inside the Earth by the decomposition of petrified animals and plants.
The plants were eaten by animals, and the plants made food for themselves through the process of photosynthesis. In photosynthesis, the energy is derived by the Sun, so the Sun's energy is converted into fossil fuels by various processes.
Thus, the ultimate source of energy in fossil fuels is the Sun.