Which gas is commonly known as "marsh gas"?

Marsh gas is commonly recognized as methane, which is a colorless, odorless gas that occurs naturally in wetlands and marshy areas. This name arises because methane is produced during the anaerobic decomposition of organic matter in these environments, where bacteria break down decay without oxygen, leading to the generation of this gas. Methane is also a significant component of natural gas, which is used as fuel and for various other applications.

In contrast, the other gases listed do not share the same characteristics or origins. Nitrogen dioxide is a reddish-brown gas that is a significant air pollutant, particularly from vehicle emissions. Hydrogen sulfide has a characteristic foul odor of rotten eggs and is typically produced by the decay of organic matter in the absence of oxygen, particularly in certain industrial and sewage contexts. Carbon dioxide is a colorless gas vital for respiration and photosynthesis but does not have the connection to marshy areas and organic decomposition that gives methane the name "marsh gas." Thus, methane stands out clearly as the correct answer in this context.