What gas is characterized by a smell resembling rotten eggs?

The gas characterized by a smell resembling rotten eggs is hydrogen sulfide. This gas naturally occurs from the decomposition of organic matter and is well known for its distinctive odor. Hydrogen sulfide is often associated with sewage, volcanic gases, and petroleum production. Because of its hazardous nature and ability to pose significant health risks (including being toxic and potentially lethal in high concentrations), this distinctive smell serves as an important warning sign to individuals in environments where it may be present.

In contrast, while methane and natural gas are both odorless in their pure forms, they are often treated with an odorant, typically mercaptan, which gives them a scent reminiscent of rotten eggs for safety reasons. Carbon dioxide, on the other hand, is also odorless and does not possess a smell at all. Therefore, among the options provided, hydrogen sulfide most accurately reflects the characteristic odor described in the question.