Which one of the following gas smells like a rotten egg?

Hydrogen sulphide (H2S) is a colorless, flammable, and highly toxic gas characterized by a distinct odor of rotten eggs [1]. This smell is detectable even at very low concentrations, though prolonged exposure or high concentrations can lead to olfactory fatigue, where the sense of smell is lost. It occurs naturally through the bacterial decomposition of organic matter in environments like swamps, sewers, and salt marshes [1]. In contrast, other options have different olfactory profiles: Ammonia has a sharp, pungent odor; Sulphur dioxide (SO2) is associated with the smell of burning sulphur or a choking, pungent odor [4]; and Acetylene is typically odorless in its pure form but may have a garlic-like smell due to impurities. Hydrogen sulphide is often referred to as 'sewer gas' or 'stink damp' because of its foul, egg-like scent.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > Iron + Dilute Hydrochloric acid Iron chloride + Hydrogen gas > p. 128
2. [4] https://wwwn.cdc.gov/TSP/MMG/MMGDetails.aspx?mmgid=385&toxid=67