Final answer:
The correct answer to the student's question is (a) Neurons in the CNS perform these functions, since the various roles specified are mainly carried out by different types of glial cells in the CNS, not exclusively by ependymal cells.
Step-by-step explanation:
The functions listed in the student's question pertain to the roles of various glial cells in the central nervous system (CNS) and the peripheral nervous system (PNS). Specifically, astrocytes are responsible for maintaining the extracellular fluid composition, providing supportive framework, and assisting with the blood-brain barrier. Ependymal cells are involved in the secretion of cerebrospinal fluid (CSF). It is not accurate to say that only ependymal cells perform all these functions. Therefore, the correct answer to the question is (a) Neurons in the CNS perform these functions.