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

1. The question asks which chemical weapon was used in the First World War. 2. Several chemical agents were used during WWI, including chlorine gas, phosgene, and mustard gas. 3. Mustard gas (sulfur mustard) was first used effectively by the German army in 1917. It is a blistering agent that caused severe burns to the skin, eyes, and respiratory tract. 4. Carbon monoxide is a toxic gas but was not typically deployed as a weapon in WWI. 5. Hydrogen cyanide was used, but it was less effective than other agents. 6. Water gas is a fuel gas mixture, not primarily a chemical weapon.