B. Barrier reef . A barrier reef is a coral reef separated from the land mass by a large lagoon with comparatively deep water. Barrier reefs occur in association with continental land masses. Corals facing the open ocean get more food and oxygen than those on the shore side. Eventually the back ones die and the reef grows outward from the shore and forms a lagoon behind it.
Barrier reefs can extend all along a coast if the water is warm enough to support coral growth.
The Great Barrier Reef of Australia extends for 1,200 miles off the coast of Queensland. Second largest is off Yucatan coast of Mexico and Belize. Probably grew due to rising ocean levels. Weight causes this. An atoll is an isolated ring of coral islands and reefs that nearly, or entirely, enclose a lagoon.