Pearl is the secretion of
A. Mouth
B. Mantle
C. Kidney
D. None

Hint: In the pearl industry, pearl oysters produce a good quality pearl with the help of secretions released by the dorsal body wall of the molluscs. It covers the visceral mass and protrudes in the form of flaps beyond the visceral mass.

Complete answer:
Molluscs are commonly found in the sea, freshwater, and land. These organisms are bilaterally symmetrical. The body of some organisms become asymmetrical due to torsion. Organisms such as oysters and clams belong to this phylum. If any foreign substance such as sand particles or a parasite enters the shell and the mantle, a layer of pearl is secreted which is known as the nacre. It is secreted by the mantle cavity to prevent irritation. Many other layers are secreted around the nacre which finally forms the pearl. Best quality pearls are secreted by the commonly known pearl oyster which is used for the commercial production of pearls. Its scientific name is Pinctada vulgaris. Pearl with superior quality is produced by the pearl oyster in about seven years. Freshwater and saltwater pearls may sometimes look quite similar, but they come from different sources.

Hence, the correct answer is option B.

Note: In some molluscs, the visceral mass and foot rotate by 180 degrees. It occurs in all gastropods in their larval development. This process of rotation makes the body of some molluscs asymmetrical. The mantle cavity in these molluscs helps in the secretion of pearls.