Final answer:
The central axis around which a snail's shell coils is known as the columella, and this structure is integral to the snail's shell formation. The correct option is b.
Step-by-step explanation:
The central axis around which the shell of a snail coils is called the columella. This structure is fundamental to the anatomy of snails, which are a group of mollusks known as gastropods.
Unlike the radula, which is a specialized organ mollusks use for feeding, the columella serves as the central support structure around which the snail's shell spirals.
The mantle, another anatomical feature of mollusks, is the tissue layer that secretes the shell, while the operculum is a hard protective cover found in some gastropods that closes the opening of their shells when they retract their body inside. Option b. is the correct one.