The correct answer is D) Foam. Foam is the most effective agent to use for extinguishing a fire in drums of flammable liquids stowed on the weather deck of a vessel. Foam works by creating a blanket that smothers the fire, preventing oxygen from reaching the fuel source and cooling the surrounding area. This makes it highly effective against flammable liquid fires, which can quickly spread and are difficult to extinguish with other agents like water or carbon dioxide. The other options would be less effective. Carbon dioxide (A) lacks the cooling properties needed for this type of fire, while steam (B) and water fog (C) do not provide the same smothering and sealing effect as foam.