Option 1: Carbon monoxide wasn`t used as a chemical weapon in World War I. While it`s a dangerous gas due to its ability to bind with hemoglobin in blood and prevent oxygen transportation, it was not deployed in war.

Option 2: Hydrogen cyanide, though lethal, was not used as a chemical weapon in the First World War. It was later used in World War II in gas chambers.

Option 3: Mustard gas was extensively used during World War I as a chemical warfare agent. It caused blisters, blindness, and severe internal and external injuries, making it a deadly weapon. This is the correct answer.

Option 4: Water gas is a mixture of carbon monoxide and hydrogen. It is not toxic enough to be used as a weapon and was not deployed in World War I. It`s primarily an industrial fuel.