The correct answer is C. Zinc.
Galvanization is the process of applying a protective zinc coating to iron or steel to prevent rusting. This is done by dipping the iron or steel in a molten bath of zinc. The zinc coating forms a barrier between the iron or steel and the environment, preventing oxygen and moisture from reaching the metal and causing it to rust.
Copper, lead, and mercury are not suitable for galvanizing iron or steel. Copper is a relatively expensive metal, and it is not as effective as zinc at preventing rust. Lead is a soft metal that is easily damaged, and it is also a health hazard. Mercury is a highly toxic metal that is not suitable for use in any application where there is a risk of exposure to humans or the environment.
Zinc is the most common metal used for galvanizing iron or steel. It is a relatively inexpensive metal, and it is very effective at preventing rust. Zinc is also a non-toxic metal that is safe to use in most applications.