Answer:
During combustion, the fuels chemical energy is transformed to thermal energy.
Fossil fuels contain energy that came from the sun. In fact, the sun is the source of energy for most of Earths processes. Within the dense core of the sun, during the process of nuclear fusion, nuclear energy is transformed to electromagnetic energy as well as other forms. Some of this electromagnetic energy reached Earth in the form of light.
When the suns energy reaches Earth certain living things—plants, algae, and certain bacteria—transform some of it to chemical energy. The rest is stored.
Fossil fuels can be burned to release the chemical energy stored millions of years ago. This process of burning fuels is known as combustion.