The correct answer is C. Mustard gas.
Mustard gas is a chemical weapon that was used extensively during World War I. It is a blistering agent that causes severe burns to the skin, eyes, and respiratory system. It can also cause death.
Carbon monoxide is a colorless, odorless gas that is produced when organic matter is burned. It is a respiratory poison that can cause death by asphyxiation.
Hydrogen cyanide is a colorless, flammable gas that is used in chemical weapons and insecticides. It is a respiratory poison that can cause death by asphyxiation.
Burning vapor is a term used to describe the gaseous products of combustion. It can be composed of a variety of chemicals, depending on the material that is being burned. Burning vapor can be toxic and/or flammable.
In conclusion, mustard gas was the chemical weapon that was utilized during World War I. It is a blistering agent that causes severe burns to the skin, eyes, and respiratory system. It can also cause death.