The correct response is:
High water temperatures and water acidity cause coral polyps to expel algae, causing them to turn white in color.
Coral bleaching occurs when corals experience stress from factors like elevated water temperatures and increased acidity, leading them to expel the symbiotic algae (zooxanthellae) that live in their tissues. This loss causes the corals to turn white, as the brightly colored algae are no longer present.