Answer:
b) It started as a fringing reef
Explanation:
When an island (basically a seamount or eroded volcano) that is formed on a caldera and which is surrounded by fringing reefs starts to sink or the sea level starts to rise gradually and engulf it , i.e. when the fringing reefs starts to grow it gradually forms an atoll .
It is basically a coral reef encircling a logoon . Coral starts to grow and build a fringing reef and later this barrier reef forms the atoll , which encircles a lagoon .
