During First World War which of the following gas was used as a chemical weapon ?

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