The correct answer is A. CH4. Marsh gas is a naturally occurring gas that is found in swamps and marshes. It is composed primarily of methane, which is a colorless, odorless gas that is lighter than air. Methane is produced by the decomposition of organic matter, such as plants and animals, in the absence of oxygen. It is also a major component of natural gas.
Option B, C2H6, is ethane. Ethane is a colorless, flammable gas that is lighter than air. It is a major component of natural gas and is also used as a fuel in industry.
Option C, H2S, is hydrogen sulfide. Hydrogen sulfide is a colorless, flammable gas that has a rotten egg smell. It is toxic and can be fatal if inhaled in high concentrations.
Option D, C2H2, is acetylene. Acetylene is a colorless, flammable gas that is lighter than air. It is used as a fuel in welding and cutting torches.
In conclusion, the correct answer is A. CH4. Marsh gas is primarily composed of methane.