Which among the following is a cleaner fuel?

CNG is a cleaner fuel.

CNG(Compressed Natural Gas) produces harmful products in very small amounts. It is a cleaner fuel because it causes less pollution by emitting fewer pollutants like carbon dioxide, sulphur dioxide, nitrogen dioxide, particulate matter, unburned hydrocarbons, etc. Petrol, diesel, and wood release high amounts of pollutants during the combustion process.