Copper becomes green when exposed to moist air for a long period. This is due to :

Copper becomes green when exposed to moist air for a long period. This is due to the formation of a layer of basic carbonate of copper on the surface of copper.

When copper metal is exposed to air, it becomes green due to corrosion. In the rainy season, when a copper vessel is exposed to air, the metal interacts with gases, moisture, and atmospheric gases to generate a combination of copper carbonate and copper hydroxide. This causes the surface of copper metal to become green.

The reaction is as follows:

$2\mathrm{Cu}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2+{\mathrm{O}}_2\to \mathrm{Cu}(\mathrm{OH}{)}_2+{\mathrm{CuCO}}_3$

As a result of the creation of a layer of basic carbonate of copper on the surface of copper when exposed to wet air for an extended length of time, copper becomes green.

Therefore, option D is correct.