The correct answer is (d), mustard gas.
Mustard gas is a chemical weapon that was used extensively during World War I. It is a blistering agent that causes severe skin burns, blindness, and respiratory problems. It is also a mutagen and carcinogen, meaning that it can cause genetic mutations and cancer.
Carbon monoxide is a colorless, odorless gas that is produced by incomplete combustion of carbon-containing materials. It is a respiratory poison that can cause death by asphyxiation.
Hydrogen cyanide is a colorless, flammable gas with a bitter almond odor. It is a highly toxic chemical that can cause death by asphyxiation.
Phosgene is a colorless, odorless gas that is produced by the reaction of carbon monoxide and chlorine. It is a lung irritant that can cause death by asphyxiation.
All of these gases are dangerous and can cause serious health problems. However, mustard gas is the only one that was specifically used as a chemical weapon during World War I.