A is the portion of the coral that has developed into a reef over thousands of years. The correct option is D.
What is coral reef?
Coral reef is defined as an undersea habitat distinguished by coral reef-building. Coral polyp colonies are bound together by calcium carbonate to build reefs. They provide meals and novel medications.
The calcium carbonate skeleton of the coral polyps serves as the foundation for the reef. The coral polyps steadily add layers of limestone to their skeletons over the course of hundreds or thousands of years, growing outward and upward and extending the reef.
Thus, A is the portion of the coral that has developed into a reef over thousands of years. The correct option is D.
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