Concept:Mustard gas (sulfur mustard) is an organic compound containing two chloroethyl groups attached to a sulfur atom.Explanation:Option A: COCl2 is phosgene, not mustard gas.Option B: Cl−CH2−CH2−S−CH2−CH2−Cl is the correct structure of mustard gas.Option C: CCl3NO2 is chloropicrin, not mustard gas.Option D: CCl2F2 is Freon-12, a refrigerant, not mustard gas.Therefore, the correct option is B.Answer:Option B: Cl−CH2−CH2−S−CH2−CH2−Cl