Question

cnidaria - shape of life
what do coral polyps collectively build?
○ rocks
○ reefs
○ shells
○ islands

Explanation:

Coral polyps are small marine animals. They secrete calcium carbonate. Over time, the accumulated calcium carbonate structures formed by many coral polyps result in reefs. Rocks are natural formations not primarily built by coral polyps. Shells are protective outer layers of some mollusks. Islands can be formed by various geological processes like volcanic activity or sediment deposition, not mainly by coral polyps' collective building.

Answer:

Reefs