Coral islands are made of a. the remains of small marine organisms b. volcanic magma and ash c. worn-down mountains d. sand and rock

A. the remains of small marine organisms

Explanation:

   Coral islands or coralline islands, as their name implies, are made up of coral reefs. It is a less common type of training. Its development occurs when coral reefs settle on submerged rocks. In these rocks, the corals deposit their skeletons and give rise to the so-called coral rocks. They form the reefs and also the circular-shaped islands known as atolls.