After being exposed to air for a while, a layer of green basic carbonate forms on a metal; that metal is:

Correct answer: (a) Copper

Explanation

In moist air, copper slowly gains a green coating of basic copper carbonate, CuCO₃·Cu(OH)₂, formed with carbon dioxide, oxygen and water.

This green layer is the patina (verdigris) seen on old copper roofs, utensils and statues.