Final answer:
The majority of internal parasites impacting horses fall under the category of Nematodes, which include various types of common intestinal worms.
Step-by-step explanation:
Most internal parasites that affect horses are classified as: a) Nematodes b) Arthropods c) Flatworms d) Protozoa. The correct answer is a) Nematodes. These are common intestinal parasites and include roundworms, pinworms, hookworms, and whipworms. Helminths such as nematodes are widespread and can be divided into three groups, which also include cestodes (tapeworms) and trematodes (flukes). Nematodes are particularly noteworthy because they represent a large number of parasitic species with significant impacts on animal health.