Which of the following gases is considered highly explosive when present in air?

Methane is considered highly explosive when present in air due to its flammability and the specific concentration range that can lead to an explosive atmosphere. Methane is a hydrocarbon gas that ignites easily when mixed with air in the right concentrations, typically between about 5% and 15% by volume. This mixture can become highly dangerous as it can ignite from a spark or flame, leading to an explosion.

In contrast, other gases mentioned do not share this characteristic to the same extent. Carbon dioxide is not flammable and does not support combustion; nitrogen dioxide is a toxic gas but not typically classified as highly explosive; hydrogen sulfide, while toxic and flammable, generally has a narrower explosive range compared to methane. Thus, methane's properties make it significantly more hazardous in terms of explosion risks in air.