What is the colour of the component formed when copper carbonate decomposes?

Few substances decompose when heated and produce two or more products from the reactant. Thermal decomposition is the name given to this type of heating-induced breakdown. Heat causes the reactants' hue to vary from that of the products generated during thermal breakdown.

When Copper Carbonate $\left({\mathrm{CuCO}}_3\right)$ which is green in colour is heated, it decomposes and forms the end products which are Copper Oxide$\left(\mathrm{CuO}\right)$ and Carbon dioxide$\left({\mathrm{CO}}_2\right)$. 

This reaction is a decomposition Reaction. While heating ${\mathrm{CuCO}}_3$, a brisk effervescence of ${\mathrm{CO}}_2$ gas is observed. 

A decomposition Reaction occurs when a pure substance is heated and its particles/components rearrange into two or more new products.

The reaction for this reaction is:

${\mathrm{CuCO}}_3\to \mathrm{CuO} + {\mathrm{CO}}_2 \left(\mathrm{g}\right)$

Hence, the compound CuCO₃ changes its colour from light greenish colour to black colour.

Hence, option B is correct.