An atoll is best described as:

• A. A ring-shaped island formed by a collapsing volcanic island.
• B. A ring-shaped island surrounding a central lagoon that forms as a volcanic island subsides.
• C. A long, sandy strip parallel to the coast.
• D. A submerged ring of corals offshore.

Answer: (B)

An atoll is a ring-shaped coral reef that encloses a central lagoon. It forms through a sequence where a volcanic island rises, the reef builds around it, and as the island slowly subsides, the reef continues to grow upward. Eventually the island sinks below sea level, leaving a complete ring of coral with a lagoon in the middle. This description fits the idea of a ring-shaped island surrounding a central lagoon that forms as the volcanic island subsides. The other options describe different coastal features—such as a spit or a fully submerged ring—which do not match the characteristic lagoon-centered ring of an atoll.