Which of the following is an example of fossil fuel?

The correct option is D Natural gas
Fossil fuel is a fuel formed by natural processes, such as the anaerobic decomposition of buried dead organisms, containing energy originating in ancient photosynthesis.

Natural gas is a mixture of gases that are rich in hydrocarbons. All these gases are naturally found in the atmosphere.

Natural gas reserves are deep inside the earth and are fossil fuels. coal gas, coke, and producer gas are the derivatives of coal, not naturally occurring.

Hence, option $C$ is correct.