Are coral islands consisting of reefs surrounding a lagoon.

An atoll. An atoll is a ring shaped reef, island or chain of islands formed by coral.

Coral atoll

Explanation:

A coral island is an island that is formed by organic matter and occurs in tropical and subtropical areas. Coral reefs forms around the volcanic hotspots as the ocean undergoes submergence. The atoll is a ring-shaped coral reef that surrounds a lagoon partially or completely. As it forms over the volcanic creator and hence prevents the water from entering the lagoon.

Answer (d) may increase or decrease porosity through dissolution or deposition;