Final answer:
Phosgene is classified as a division 1 very toxic substance due to the serious and often fatal effects it has upon exposure.
Step-by-step explanation:
Phosgene is a division 1 very toxic substance. This assessment of its toxicity is based on historical and modern evaluations of its use and effects.
During World War I, phosgene was used as a chemical weapon and it accounted for a large percentage of the deaths caused by gassing during that time. Phosgene gas is colorless and highly lethal; inhalation can lead to severe lung damage and death. Unlike organophosphates such as parathion, which are also highly toxic but degrade quickly in the environment, phosgene's acute toxicity poses significant immediate dangers.
Therefore, the correct option is A) very toxic.