Which of the following is considered a shellfish?

Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater.

Molluscs include cephalopods (squids, octopuses, cuttlefish) and bivalves (clams, oysters), as well as gastropods. They are used as a food source by humans include many species of clams, mussels, oysters, winkles, and scallops. Some crustaceans that are commonly eaten are shrimp, lobsters, crayfish, and crabs. Echinoderms are not as frequently harvested for food as molluscs and crustaceans; however, sea urchin roe is quite popular in many parts of the world, where the live delicacy is harder to transport.