Answer: a. skeletons of coral animals
A coral reef is a deep underwater ecosystem exhibiting walls or reefs build by corals. These reefs are formed by the colonies of organisms called as polyps, which are held together by calcium carbonate. The coral reefs are build in order to form stony corals, because of clustering of the polyps. The hard skeletonized structure provide protection to the polyps. These also provide shelter to small marine organisms like fishes.