In Mollusca, shell is secreted by
a. Mantle
b. Maize
c. Wheat
d. Osphradia

Hint: Mollusca is a phylum which falls under kingdom Animalia. It is the second largest phylum which falls below the largest phylum Arthropoda. Molluscs are soft bodied animals covered by shells. It is a significant part of their anatomy.

Complete answer:
Mollusca is the second largest phylum which falls under kingdom Animalia. Molluscs are soft bodied animals covered by a hard shell made of calcium carbonate. With time and evolution, they lost the complete and it was replaced by mantle, a fold of tissues draped over the visceral mass and secretes a shell. This mantle has a cavity which encloses water filled chambers. These chambers include anus, gills, and excretory organs. It also functions as respiratory chambers in most molluscs. It is highly vascular.

The mantle secreted shell has bigger size and more strength. The shell is secreted by epithelial cells of the mantle which is an ectoderm layer. The mantle protects the internal organs, it helps in respiration, feeding, excretion, locomotion, it filters food.
Examples of mollusks with shells- snails, limpets (gastropods; have only one shell), clams, oysters, scallops (bivalves; have two shells)

Note all molluscs have shells such as squids, slugs, octopus (cephalopods; keep their shells inside of their bodies). The only cephalopod that has an external shell is Nautilus.

Hence, the correct answer is option (a).

Note: Molluscs have about 85,000 recognized species. They are considered to be the largest marine phylum. Although phylum Arthropoda is the most advanced group. Some molluscs are used as food. Molluscs oysters and mussels produce pearls which are deemed to be precious.