Answer: The correct answer is-
d. Platyhelminthes.
Platyhelminthes is a phylum of invertebrates that includes flatworms such as Tapeworms, Flukes. Most of the members that act like parasite belong to the class Tremada ( which includes flukes) and Cestoda ( which includes tapeworms) within this phylum.
They can parasitize human beings ( for example- Fascioliasis is an infectious disease caused by Fasciola hepatica, which is commonly called liver fluke).
Thus, option d) is the right answer.