An atoll is a:

Correct answer: A ring-shaped coral reef. An atoll is a ring-shaped coral reef or chain of islands that surrounds a lagoon.

Background: Darwin explained atolls as coral growing around a sinking volcanic island.

• Racket cave: “Racket cave” is not a real landform.
• Coral reef: “Coral reef” is a general term, while an atoll is a specific ring-shaped reef.
• None of these: “None of these” is incorrect because a ring-shaped coral reef is correct.

Did you know? The Maldives is made up of atolls.

Memory trick: Atoll = ring of coral.