Answer: A. Bacteria
Step-by-step explanation:
A biotic factor is one of the important component of the ecosystem other one is abiotic. A biotic factor includes all living beings such as plants, animals, humans and others.
The coral reef is formed by the calcium carbonate secreted by the coral polyps.
The bacteria is the correct option this is because of the fact that bacteria is a living being which can live in symbiotic association with the coral polyps.