Brass gets discoloured in air because of the presence of which of the following gases in air

Brass which is an alloy of copper and zinc gets discoloured in air due to the presence of hydrogen sulphide gas. When hydrogen sulphide comes in contact with brass it reacts with copper to form copper sulphide which is black in color. This chemical reaction creates a tarnished layer on the surface of brass. Over time the shine of brass decreases and it looks dull. This process is called tarnishing and is common for copper and brass materials. To prevent tarnishing brass objects are polished regularly or coated with protective layers. Therefore hydrogen sulphide gas is the main cause of discoloration.