Corals is/are

Answer: (d) The remains of tiny sea animals resembling shells

The hard coral we see is the limestone skeleton left by tiny polyps — the remains of sea animals.

The idea

A coral is a tiny, soft animal called a polyp, related to the jellyfish. It takes lime from sea water and builds a hard cup around itself. Millions of polyps live joined together, and new ones build on the skeletons of the dead. Over thousands of years this makes a reef.

Polyp builds a limestone cup → Tiny algae live inside the polyp and feed it → Colony grows → Old polyps die, skeletons remain → A reef rises

Why the others are wrong

(a) Corals are not shells.

(b) Corals are not fish.

(c) Living coral polyps are tiny animals, but they build limestone cups rather than live in shells — this wording describes a mollusc.

The trap

(c) sounds close to the living polyp. The question means coral as the material.

Also know

The algae (zooxanthellae) also give the coral its colour. When the water gets too warm, the coral throws them out, turns white and may starve. This is coral bleaching.