Which one of the following was used as a chemical weapon in the First World War?

The chemical agent used in the First World War among the options is mustard gas (sulfur mustard). Historical records identify sulfur mustard—commonly called mustard gas—as one of the most widely used chemical warfare agents during World War I, first employed in the Flanders/Ypres area in July 1917 and subsequently used in later actions and conflicts [1]. While other gases (for example, phosgene) caused a larger share of fatalities, mustard gas was notorious for its persistent blistering effects, chronic respiratory injury and long-term environmental residues from WWI-era use and stockpiles [2]. Thus, of the listed choices, mustard gas is the correct answer.

Sources

1. [1] https://publications.iarc.who.int/_publications/media/download/5297/866be849cedf91b8d4c787bccea91c37ce741d8c.pdf
2. [2] https://www.aaas.org/taxonomy/term/10/vestiges-chemical-warfare