Final answer:
The correct answer is planulae. The three major structures of coral reefs are fringing reefs, barrier reefs, and atolls.
Step-by-step explanation:
The correct answer is planulae. The three major structures of coral reefs are fringing reefs, barrier reefs, and atolls. Fringing reefs are located close to the shore and are directly attached to it, while barrier reefs are separated from the shore by a lagoon. Atolls are circular reefs that are often formed around a sunken volcano. Planulae, on the other hand, is the larval stage of a coral polyp before it settles on a substrate to start building a reef.
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