Final answer:
Tapeworms are the endoparasites closely linked to flea infestations. They attach themselves to the intestinal tract of vertebrates, including fleas, and absorb nutrients from the food matter passing through the host's intestine.
Step-by-step explanation:
The endoparasite closely linked to flea infestations is B) Tapeworms. Tapeworms are internal parasites that mainly infest the intestinal tract of vertebrates, including fleas. They attach themselves using a on the anterior end, called a scolex, and absorb nutrients from the food matter passing through the host's intestine. Fleas can become infected with tapeworm larvae by consuming tapeworm eggs, and when a pet ingests an infected flea during grooming, they can become the host for tapeworms.