Solution

Pearl is secreted by the mantle of the pearl oyster.
Nacreous layer is the innermost layer of the shell and is better known as the mother of pearl.
This layer is responsible for the formation of pearl. It consists of alternate layers of calcium carbonate and conchiolin, which lie parallel to the surface of the shell.
Any sort of foreign body enters and gets between the mantle and shell, it becomes enclosed in a sac of mantle epithelium.
This foreign body now acts as an irritant and stimulates the mantle epithelium to secrete concentric layers of nacre around this foreign body which hardens and forms the pearl.
Thus, the correct answer is option B.