Which of the following requirements results virtually limits the area distribution of reef building corals to the tropical and sub-tropical zones?

The requirement that virtually limits the area distribution of reef building corals to the tropical and sub-tropical zones is a mean water temperature not less than 20°C. This means that coral reefs require warm water to survive and thrive. Cold water can be harmful to corals and cause them to die off. Tropical and sub-tropical zones have the necessary warm water temperatures that are required for the growth and survival of coral reefs. Therefore, coral reefs are mainly found in these areas. The other options, such as warm currents, shallow water of less than 30 meters, and saline but sediments-free water, are also important for the growth and survival of coral reefs but are not the main factor that limits their distribution to tropical and sub-tropical zones.