The algae that live inside coral polyps and give reefs their colour are called:

The correct answer is B, zooxanthellae. These single-celled algae live within the tissue of the coral polyp, photosynthesise, hand most of the food they make to the polyp and give the reef its colour; the polyp in return offers shelter and nutrients, a textbook case of symbiosis. When heat stress makes the polyp expel them the reef turns white, which is coral bleaching. Option A is wrong because diatoms are free-floating algae with silica walls, an important part of marine plankton but not reef symbionts. Option C is wrong because Sargassum is a large brown seaweed that floats in mats, best known from the Sargasso Sea. Option D is wrong because Chlorella is a green alga of fresh water, grown for food supplements and used in laboratory work. Bleaching questions always begin from this partnership, so fix the name firmly.