Correct Answer: hydrogen sulphide

Solution : The correct option is Hydrogen Suphide.

Brass may get discolour when exposed to hydrogen sulphide (H2S) gas. In certain environments, this is a common cause of tarnishing or discoloration of brass. Hydrogen sulphide is a gas that can be found in the environment, particularly in industrial locations or areas with high levels of pollution. When exposed to hydrogen sulphide, brass can react with the gas, forming a coating of black copper sulphide (CuS) on the surface. This black coating can give brass a dark and discoloured look that differs from the typical golden colour of brass.