Fossils are the remains of organisms that are buried in a rock during sediment deposition over long geological time period. Since the bodies of these organisms is made of organic compounds, the high pressure of the overlying rock turns them into the hydrocarbons or fossil fuels. Therefore, option D is correct.
On the other hand, options A-C are incorrect because of following reasons:
Burning of natural gas doesn't form fossil fuel but CO2. In fact, natural gas is also a form of fossil fuel which is released from a rock hosting fossils.
Sunlight doesn't convert grass to coal. In fact, sunlight cannot penetrate deeper into the geological rocks. Rather it will cause the weathering of surface sediments directly exposed to sunlight.
Farms are not used to produce natural gas rather natural gas is produced inside a rock hosting bodies of living organisms.
