Which of the following helminths should be classified as nematodes?

Nematodes, commonly known as roundworms, are characterized by their elongated, cylindrical shape and full digestive tract. When considering the options provided, pinworm fits the classification of a nematode because it is a specific type of roundworm that belongs to the Enterobius genus. This parasitic worm primarily infects the intestines of humans, particularly in children, and causes symptoms such as itching around the anus.

Other choices include tapeworms, which are flat, segmented worms belonging to the class Cestoda, and flukes, which are also flatworms but belong to the class Trematoda. These distinct classifications differentiate them from nematodes due to their unique morphological and biological characteristics. Roundworm, while it might seem relevant, is a general term that can refer to various species within the nematode category, making pinworm the more specific answer in this context.