Final answer:
The apex is the oldest part of the shell of a mollusk, and its growth can be observed in the concentric rings that form around it. Organisms like dolphins have fusiform shapes, and the exoskeleton refers to a hard protective outer layer.
Step-by-step explanation:
The apex is the oldest part of the shell, and growth occurs in concentric rings around it. The correct answer to the given question is therefore a. Apex. Exploring further, growth rings in shells can provide insight into the age of a mollusk, similar to counting the rings in the cross-section of a tree.
Layers of calcium carbonate and other organic materials accumulate around the apex as the mollusk ages, forming a sizeable protective shell. Besides apex, other keywords that appear in our exploration of mollusks include exoskeleton, which is the term for a hard covering or shell that provides protection and muscle attachment, and fusiform, a shape exhibited by organisms like dolphins, characterized by a spindle-shaped body that is tapered at both ends, which is effective for moving through water.