Final answer:
Cephalopods are the class of molluscs that can lack a shell, with examples including squids and octopuses, unlike chitons, most gastropods, and bivalves, which possess external shells.
Step-by-step explanation:
The Mollusc Without a Shell
Among molluscs, there is a diversity of forms, which includes the presence or absence of a shell. When considering molluscs that lack a shell, we look to the class Gastropoda and within that, specifically to slugs. While the majority of gastropods, like snails, carry a shell, slugs are the gastropods that have a reduced or absent shell. Similarly, the classes Caudofoveata and Solenogastres also consist of shell-less, worm-like animals, but these weren't options provided in the question.
Cephalopods represent another class within the Phylum Mollusca where the shell is either internally reduced or absent. Animals in this class, like squids and octopuses, have evolved without an external shell, allowing them to have more flexible bodies ideal for their predatory lifestyle and complex behaviors. However, some cephalopods, such as the nautilus, do retain a prominent external shell. Meanwhile, Chitons (Polyplacophora) and Bivalves like clams and oysters maintain their external shells, which is crucial for their protection.
Considering the options given: chitons, gastropods, bivalves, and cephalopods, the correct answer is that cephalopods are the molluscs that can lack a shell, particularly notable in species such as squids and octopuses.