The correct answer is D. Octopus is a mollusc.
Molluscs are a large and diverse group of invertebrates 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 they all share some common features, such as a radula, a foot, and a mantle.
The radula is a tongue-like organ that is used for scraping food. The foot is a muscular organ that is used for locomotion. The mantle is a sheet of tissue that covers the body and secretes the shell.
Molluscs are found in all aquatic environments, as well as on land. They are an important part of the marine food web, and they are also a source of food for humans.
Option A: Arthropods are invertebrates that have a segmented body, jointed appendages, and an exoskeleton. They include insects, spiders, crustaceans, and millipedes.
Option B: Echinoderms are invertebrates that have a radial body plan, an endoskeleton, and tube feet. They include starfish, sea urchins, and sand dollars.
Option C: Hemichordates are invertebrates that have a notochord, a hollow nerve cord, and gill slits. They include acorn worms and pterobranchs.