Metal P is zinc; Metal Q is tin; Metal R is lead; Metal S is iron.
Metal P (zinc) is used to form a thin layer on metal S (iron) by the process of galvanisation to prevent its corrosion.
Metal Q (tin) is used for electroplating tiffin boxes made of metal S (iron).
Metal R (lead) is used in making car batteries.
Metals Q (tin) and R (lead) form an alloy called solder.