Coral reefs that are preserved as sedimentary rocks are classified as

The answer is; fossil

Fossil is defined as any dead organism (or its trace or impression) of a past geological time that is naturally preserved in the earth in geological features such as rocks, bogs, and etcetera. Coral reef is no exception because it is considered a living organism. It is composed of colonies of coral polyps and algae that secrete a carbon carbonate skeleton.