The correct answer is A. Methane.
Marsh gas is a naturally occurring gas that is found in wetlands and swamps. It is composed primarily of methane, which is a colorless, odorless, and flammable gas. Marsh gas is formed by the decomposition of organic matter, such as plants and animals, in the absence of oxygen. It can also be produced by human activities, such as the decomposition of sewage and garbage.
Methane is a greenhouse gas that contributes to climate change. It is also a major contributor to air pollution. Marsh gas can be a fire hazard, and it can also be a health hazard if it is inhaled.
Ethane, propane, and butane are also flammable gases, but they are not typically found in marshes. Ethane is a colorless, odorless gas that is used as a fuel and a chemical feedstock. Propane is a colorless, odorless gas that is used as a fuel for heating and cooking. Butane is a colorless, odorless gas that is used as a fuel for lighters and camping stoves.