The Correct answer is Methane. Explanation & Key Takeaways: • Natural gas (also called fossil gas; sometimes just gas), is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carbon dioxide, nitrogen, hydrogen sulfide, or helium. • It is formed when layers of decomposing plant and animal matter are exposed to intense heat and pressure under the surface of the Earth over millions of years. • The energy that the plants originally obtained from the sun is stored in the form of chemical bonds in the gas. • Natural gas is a fossil fuel. It is a non-renewable hydrocarbon used as a source of energy for heating, cooking, and electricity generation. • It is also used as a fuel for vehicles and as a chemical feedstock in the manufacture of plastics and other commercially important organic chemicals.