a. Mangroves

b. Desert

c. Coral reefs

d. Alpine meadows

Answer: c. Coral reefs

Explanation: A coral reef is an underwater creature. Rivers are composed of colonies of coral polyps held together by calcium carbonate. Most corals are made up of stone corals and their polyps collections. Often referred to as “rain forests of the sea”, shallow coral reefs make up one of the most diverse ecosystems on Earth.