With
Coral reefs are important to ocean life because they have a
many species in the ocean.

Only hard corals buid reefs.

Explanation:

Coral Reefs

There are two types of coral: hard and soft. Only hard corals build reefs. Coral species are tiny animals that secrete an exoskeleton of calcium carbonate. When the coral polyp dies, the exoskeleton becomes part of the rocky reef. Coral reefs are some of the most highly diverse ecosystems in the world and are known as the "rainforests of the sea." While there are some deep-water corals, most corals grow in warm, shallow water, where the nutrient levels are relatively low. Many coral species have a symbiotic relationship with an algae known as zooxanthellae. These algae live within the actual coral polyp animals and can photosynthesize, providing the coral with food. Coral polyps also have tentacles they can extend to catch zooplankton in the water.

it is either c or d and i know for sure it is not a or b