The body of the __________ is circular in cross-section, hence, the name roundworms. They may be free living, aquatic and terrestrial or parasitic in plants and animals.

Correct option is D

The body of the Aschelminthes is circular in cross-section, hence, the name roundworms.
 Aschelminthes is a diverse phylum of animals that includes free-living, aquatic, terrestrial, and parasitic species.
 Some common examples of Aschelminthes include nematodes (such as the pinworm and hookworm) and rotifers.