An Atoll is _____.

The correct answer is Option (4) → A ring-shaped island with a lagoon at the centre

An atoll is a ring-shaped coral reef or chain of coral islands that encloses a shallow lagoon of sea water. It forms in warm tropical seas when coral grows around a volcanic island; as the island slowly sinks, the reef keeps growing upwards and remains as a ring with the lagoon in the middle. The Lakshadweep Islands and the Maldives are made largely of atolls.