Which clarifying sentence is most relevant to the information in the passage?

Corals are anthozoans—the largest class of organisms within the phylum Cnidaria.

A volcanic island is formed on layers of solidified lava from an undersea volcano.

Fringing and barrier reefs can be thought of as stages in the formation of atoll reefs.
Zooxanthella algae are critical elements in the continuing health of reef- building corals.