Pearls are formed inside

Answer: (c) Oysters

Pearl oysters coat an irritant with layers of nacre (mother-of-pearl).

The idea

A pearl is the oyster’s defence. When a grain or a parasite gets inside the shell and cannot be pushed out, the oyster coats it with the same shiny material that lines its shell. Layer on layer, this becomes a pearl.

Irritant enters the shell → Oyster covers it with nacre (mother-of-pearl) → Many thin layers over years → Pearl

Why the others are wrong

(a) Squids have no outer shell to make pearls in.

(b) Some snails and mussels also make true pearls; oysters are the intended answer here.

(d) Oysters are molluscs, but most molluscs don’t make pearls — the precise answer is oysters.

The trap

(d) is not false, just too broad. UPSC wants the precise answer.

Also know

In pearl culture a small bead is placed inside the oyster by hand; the method was developed in Japan. India’s pearl oysters are found in the Gulf of Mannar and the Gulf of Kachchh; Thoothukudi was known as the pearl city.