The correct answer is option A, that is, shallow and clear water.
Coral reefs refer to the shallow-ocean habitats, which are occupied with sea life. The massive composition that coral reef comprises of is actually manufactured of coral polyps that are the small marine species, which live in colonies.
The hard compositions are left behind when these marine species die, and the branching and stony composition, which is left is limestone and is robust enough to produce a home for new species.