An Atoll is _____.

An atoll is a ring-shaped coral reef, island, or series of islets that encircle a lagoon partially or completely.

Therefore, the correct option is: A ring shaped island with a lagoon at the Centre.

An atoll is a type of coral reef formation that typically occurs in warm, tropical ocean waters. It is characterized by a circular or ring-shaped structure made up of coral reefs and islets surrounding a central lagoon. Atolls are often found in areas where underwater volcanic peaks have subsided or eroded over time, leaving behind a rim of coral reefs that continues to grow and form a ring shape.