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 primarily due to stress from elevated water temperatures and changes in water chemistry (such as increased acidity), leading to the expulsion of symbiotic algae (zooxanthellae) that live within the coral's tissues. This loss of algae results in the coral's white appearance and can severely impact their health and survival.