Corals Belong to Phylum Cnidaria

Corals belong to phylum Cnidaria. The reefs themselves are made up of millions of polyps, each of which secretes a calcium carbonate skeleton that becomes part of the reef structure. Corals, which are attached to the reef as adults, have free-swimming larvae that develop into new polyps.
Corals are actually colorless. The brilliant colors that are visible on the reef come from the zooxanthellae (microscopic algae) that live within their body tissues. The zooxanthellae and corals have a symbiotic relationship in which corals provide carbon dioxide and mineral nutrients (released as wastes from coral digestion) to the zooxanthellae. The zooxanthellae perform photosynthesis. Photosynthesis produces nutrients (in the form of sugars) for the coral and also releases oxygen. This relationship supplements energy from predation and allows corals to survive in clear tropical water, even though these areas have very low nutrient levels.
-What advantage do free-swimming larvae confer on reef-building corals?

A) providing reef-building corals with a defense against fishes and other predators
B) allowing reef-building corals to establish colonies in the deep ocean
C) increasing survival since coral larvae have exoskeletons of chitin
D) allowing reef-building corals to expand their populations into new habitats