The answer is "d. increased temperature".
At the point when corals are stressed by changes in conditions, for example, temperature, light, or supplements, they remove the symbiotic algae living in their tissues, making them turn totally white. Hotter water temperatures can result in coral bleaching. At the point when water is too warm, corals will oust the algae living in their tissues, making the coral turn totally white. This is known as coral bleaching.
When a coral blanches, it isn't dead. Corals can endure a bleaching occasion, yet they are under more pressure and are liable to mortality.