The correct answer is A. Mollusca.
Molluscs are a phylum of invertebrate animals that includes snails, clams, octopuses, and squid. They are characterized by a soft body that is often protected by a hard shell. Molluscs have a variety of body plans, but all share a common ancestor.
Cnidarians are a phylum of invertebrate animals that includes jellyfish, corals, and sea anemones. They are characterized by a radial body plan and a stinging cell called a nematocyst. Cnidarians are found in all marine environments, and some species can also be found in freshwater.
Echinoderms are a phylum of invertebrate animals that includes sea stars, sea urchins, and sand dollars. They are characterized by a radial body plan and a hard skeleton made of calcium carbonate. Echinoderms are found in all marine environments.
Chordates are a phylum of vertebrate animals that includes fish, amphibians, reptiles, birds, and mammals. They are characterized by a notochord, a dorsal nerve cord, and gill slits. Chordates are found in all aquatic and terrestrial environments.
In conclusion, octopuses belong to the phylum Mollusca. They are characterized by a soft body that is often protected by a hard shell. Molluscs have a variety of body plans, but all share a common ancestor.