Question

coral reefs are most commonly found in: deep ocean rivers arctic tropical shallow waters

Explanation:

Coral reefs need sunlight for the algae (zooxanthellae) that live in their tissues. Sunlight can't penetrate deep ocean waters. Rivers have freshwater (corals are marine organisms). The Arctic is too cold. Tropical shallow waters have warm water (corals are sensitive to temperature changes) and sufficient sunlight for photosynthesis by the algae.

Answer:

Tropical shallow waters