The correct answer is: B. Hydrogen sulphide.
Brass is an alloy of copper and zinc. When brass is exposed to hydrogen sulphide, the copper in the brass reacts with the hydrogen sulphide to form copper sulphide. Copper sulphide is a black compound that forms a layer on the surface of the brass, which discolours the brass.
Aluminium phosphide, hydrogenated wafers, and aluminium sulphide are not known to cause brass to discolour.
Aluminium phosphide is a white solid that is used as a rodenticide. It is not known to react with brass.
Hydrogenated wafers are thin slices of semiconductor material that have been coated with a layer of hydrogen. They are used in electronic devices. Hydrogenated wafers are not known to react with brass.
Aluminium sulphide is a white solid that is used as a flame retardant. It is not known to react with brass.