Final answer:
The notochord's disappearance in sessile adult tunicates indicates its function as support during movement, necessary in the larval stages but not as adults. Hence, option (a) is correct.
Step-by-step explanation:
The notochord is a flexible, rod-like structure found in the embryos of all chordates, which is a phylum of animals that includes vertebrates.
The loss of the notochord in sessile adult tunicates suggests that one function of this structure is support during movement. The notochord is present in the mobile larval stages of both tunicates and other chordates, indicating its role in providing a rigid, skeletal support necessary for movement.
However, as tunicates become sessile adults, they no longer require such support, so the notochord is lost, which aligns with its function in assisting mobility rather than protection of internal organs, facilitation of filter feeding, or generation of electrical impulses.