Use reasoning to lead to the answer of the following question:
Many corals secrete what to form a coral reef?\nOptions:\n- (A) Chlorophyll\n- (B) salt and water.\n- (C) The area was once underwater\n- (D) Chemical energy\n- (E) aqueous solution\n- (F) Organic compounds\n- (G) gastropod shells\n- (H) hard outer covering

(H)
Many corals secrete an exoskeleton that builds up to become a coral reef.

An insect has a hard outer covering called an exoskeleton.

Many corals secrete a hard outer covering that builds up to become a coral reef.

.

So, the final answer is (H)