To answer this question we need to remember a bit of each group:
Arthropods: This is composed of organisms with an exoskeleton like insects or crabs.
Echinoderms: This is composed of organisms with radial symmetry like starfish.
Cnidarians: This is composed of organisms that possess cnidoblasts like jellyfish.
Mollusks: This is composed of organisms with hard shells, squids, and octopuses even when they don´t look like belong to this phylum as in the inner part of the trunk and head there is a reminiscence of the hard shell, therefore this is the correct answer.