Which coral reef type forms around a sunken island?

The formation of an atoll is a well-understood process in marine science. An atoll is a ring-shaped coral reef that forms around a sunken island, typically a volcanic island that has eroded and subsided over time. As the island sinks, corals grow upward and outward from the submerged caldera, eventually forming a circular or oval shape with a central lagoon. This unique structure is characterized by the coral growth that develops on the rim of the sunken island, while the center is generally deeper and filled with water.

In contrast, fringing reefs are directly attached to a coastline and do not form around a sunken island; they usually grow from the shore outward. Barrier reefs are separated from the shore by a lagoon but do not necessarily form around a sunken island—their development can occur along the continental shelf. Reef flats refer to the shallow areas of a reef system that are exposed at low tide, and they are not defined by their relationship with sunken landforms at all.

Thus, the atoll is uniquely defined by its development around a sunken island, making it the correct answer.