Answer: The correct answer is "igniting the gas-air mixture".
Step-by-step explanation:
Internal combustion engine is a heat engine. Here, the combustion of the fuel occurs with air.
Internal combustion is fed with fossil fuel like petroleum, natural gas product such as gasoline.
The expansion of high temperature and high pressure gases are produced by the combustion. These gases apply the direct force to some component of engine.
Therefore, the heat is supplied for the hot reservoir in car's engine by igniting the gas-air mixture.