Question

Coral bleaching occurs when the zooxanthellae algae that live in coral leave or die due to stress. Which of the following is the best evidence of a mutualistic relationship between zooxanthellae and coral rather than commensalism?

Corals are stressed and more likely to die without zooxanthellae; the zooxanthellae grow best when associated with the corals.
Corals deliberately expel the zooxanthellae when under stress.
Zooxanthellae give corals their bright colors.
Zooxanthellae provide nutrients to the corals, but corals can survive without zooxanthellae.

Answer 1

I believe the answer is that;Corals are stressed and more likely to die without zooxanthellae; the zooxanthellae grow best when associated with the corals.
The corals and the algae have a mutualistic relationship. The coral provides the algae with a protected environment and compounds they need for photosynthesis. In return, the algae produce oxygen and help the coral to remove wastes. Most importantly, zooxanthellae supply coral with glucose, glycerol, and amino acids, which are the products of photosynthesis.