The correct answer is A. Zinc.
Galvanizing is a process of applying a protective zinc coating to iron or steel to prevent rusting. Zinc is a highly reactive metal that forms a protective layer of zinc oxide when exposed to air. This layer prevents oxygen and moisture from reaching the underlying iron or steel, which helps to prevent rusting.
Mercury, cadmium, and tin are not as effective as zinc at preventing rusting. Mercury is a liquid metal that is not suitable for use in galvanizing. Cadmium is a toxic metal that is not widely used in galvanizing. Tin is a less reactive metal than zinc and does not form as effective a protective layer.
Therefore, the metal chiefly used for galvanizing iron is zinc.