The correct answer is - natural gas.
The natural gas is a fossil fuel, meaning that it is derived from the decomposition of organic matter, and through time went under certain processes in the crust that made its current composition.
This fossil fuel is known to cause much less pollution than the other fossil fuels, especially when compared to the oil and the coal which are both very big polluters.
The natural gas is distributed to the homes and factories through pipelines.
The main pollution form the natural gas doesn't come from its direct usage, but instead form the leaking during its transportation and moving it from one container to another. It releases a lot of methane in those cases which ends up in the atmosphere and contributes to the Greenhouse effect, but that is only very small amount.