Final answer:
An atoll is a ring-like coral island or string of small islands surrounding a lagoon, corresponding to the feature described in the question as a (moon-like circle in blue water). The correct answer is D) Atoll.
Step-by-step explanation:
The feature described as a (moon-like circle in blue water) is a ring-like coral island or string of small islands surrounding a lagoon. The correct answer to the question is D) Atoll. An atoll is formed from a coral reef that develops in a ring around a sunken volcanic island. Over time, the island erodes or sinks, leaving behind a lagoon that can be shallow and salty, with clear blue water due to the relatively clear and often nutrient-poor water that lagoons typically have. These conditions make atolls distinct and easily recognizable due to their ring-like shape and the stunning contrast between the deep blue ocean and the lighter blue of the lagoon.