Final answer:
Copper oxide (b) primarily causes the discoloration of copper plating. The correct option is (b).
Step-by-step explanation:
The copper compound that primarily causes the discoloration of copper plating is copper oxide (b). Copper oxide is formed when copper reacts with oxygen in the air, leading to a black or greenish-brown discoloration on the surface of copper.
For example, when a clean piece of copper metal is placed in a solution of silver nitrate, the copper metal undergoes oxidation and forms copper ions, which give the solution a blue color. During this process, silver metal is deposited on the copper strip as the silver ions are removed from the solution.