Answer is c. Natural gas.
Natural gas is a fossil fuel in liquid form that can be used as an energy source. It is found in subterranean reservoirs, often near oil deposits. It is non-renewable energy source. It takes very long time to renew. Natural gas is less pollutant as compared to coal and petroleum because when it's burned, it releases 25 % fewer greenhouse gases and pollutants. hydroelectric power, solar power and nuclear energy are renewable sources.