The correct answer is: C. Mustard gas.
Mustard gas is a chemical weapon that was used in World War I. It is a blistering agent that causes severe burns to the skin, eyes, and lungs. It can also cause death.
Carbon monoxide is a colorless, odorless gas that is produced when carbon is burned in an incomplete combustion reaction. It is a poisonous gas that can cause death by asphyxiation.
Hydrogen cyanide is a colorless, flammable gas with a bitter almond odor. It is a poisonous gas that can cause death by asphyxiation.
Water gas is a mixture of hydrogen and carbon monoxide gases that is produced by heating coal or coke in the presence of steam. It is used as a fuel and as a reducing agent.
Mustard gas was first used in World War I by the Germans in 1915. It was used by both sides in the war, and it caused a great deal of suffering. It is still used as a chemical weapon today, although it is banned by international treaty.