The correct option is Green.
Explanation
Corrosion is the gradual degradation of metals caused by the action of atmospheric gases, moisture, and other chemicals. While the corrosion of iron is specifically termed "rusting," other metals also undergo characteristic colour changes when they corrode.
Analysis of Copper Corrosion:
- When copper is exposed to moist air for an extended period, it reacts with carbon dioxide and moisture present in the atmosphere.
- This reaction causes the copper to lose its shiny brown surface and gain a green coating.
- This green substance is basic copper carbonate.
- The chemical process is represented as:
2Cu + H2O + CO2 + O2 → Cu(OH)2 + CuCO3 (Basic Copper Carbonate)
Comparison with Other Metals:
- Silver: Turns black due to the formation of silver sulfide ($Ag_2S$) upon reaction with sulfur in the air.
- Iron: Develops a reddish-brown coating called rust (hydrated iron oxide, $Fe_2O_3 \cdot xH_2O$).
Key Takeaway:
The corrosion of copper results in a green coating of basic copper carbonate, formed by the reaction with moist carbon dioxide in the air.