A combustible gas which may be present in sewer air and is explosive in the presence of oxygen is?

The correct answer to this question is methane. Methane is a colorless, odorless gas that is a major component of natural gas, and it can accumulate in sewer systems as a byproduct of the breakdown of organic matter. When methane is present in air and is mixed with oxygen in certain proportions, it can form an explosive mixture. This makes it particularly dangerous in enclosed spaces like sewers, where gas can accumulate without proper ventilation.

Ammonia, while it can be present in sewer air, is not considered explosive in typical concentrations. Carbon dioxide, although it can be hazardous in high concentrations, is not explosive nor does it support combustion. Hydrogen sulfide is indeed a toxic gas found in sewers, but it is not typically classified as explosive in the same sense as methane. Therefore, methane is the gas that poses an explosive risk in the presence of oxygen within the context of sewer systems.