The correct answer is Platyhelminthes. Explanation & Key Takeaways: • Tapeworms belong to the taxonomic class Cestoda, and they fall under the phylum Platyhelminthes, also known as the flatworms. • Tapeworms belong to this phylum, which comprises the flatworms. • Flatworms are characterized by their flat, bilaterally symmetrical bodies and lack of a body cavity (they are acoelomate). • Tapeworms, flukes, and planarians are all examples of Platyhelminthes. Related Concepts: • Mollusks: This is incorrect as mollusks are a diverse group of invertebrate taxa which include snails, clams, oysters, squids, and octopuses. They are characterized by a soft body and, often, an external or internal shell. They are distinct from tapeworms. • Nematodes: Also known as roundworms, this group includes organisms with cylindrical, unsegmented bodies that taper at both ends. They're different from tapeworms, which are flat and segmented. • Cnidaria: This phylum consists of aquatic, mostly marine species such as jellyfishes, sea anemones, and corals. They are characterized by their specialized cells called cnidocytes, which they use primarily for capturing prey. They're entirely different from tapeworms.