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

Nitrogen, oxygen, carbon dioxide, moisture, and other gases are found in air. Various products are created when some of these gases interact with copper. These items coat the copper metal's surface and give it a green tint.

The copper metal becomes green when exposed to air. Corrosion is to blame for the development of the colour green.
During the monsoon, copper metal reacts with the air to produce copper hydroxide and copper carbonate. This combination is created when copper metal reacts with water (from airborne moisture), oxygen, and carbon dioxide gases (present in the atmosphere). Copper hydroxide and copper carbonate are present in the green layer that covers the surface of copper metal. Basic carbonate is another name for the copper hydroxide and copper carbonate combo.
For the transformation of copper into a combination of copper hydroxide and copper carbonate, create a balanced chemical reaction.

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

Hence, the correct option is the option B ) formation of a layer of basic carbonate on the surface of copper.