The nodes of Ranvier are      

(a) gaps between two adjacent myelin sheaths

(b) areas where the neurilemma touches the axon

(c) the points at which axons are exposed without myelin sheath    

The nodes of Ranvier are gaps between adjacent myelin sheaths along the length of an axon, The axon membrane is exposed or unmyelinated at these nodes.

In a myelinated axon, the axon is wrapped in a fatty substance called myelin, which is produced by specialized glial cells called oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS). Myelin acts as an insulating layer that covers the axon, preventing the leakage of electrical signals (action potentials) along the axon.

However, there are gaps or interruptions in the myelin sheath at regular intervals, and these gaps are the nodes of Ranvier. At these nodes, the axon membrane is exposed to the extracellular fluid. It allows for the rapid and efficient propagation of action potentials. Action potentials "jump" from one node of Ranvier to the next, rather than traveling continuously along the entire length of the axon. This saltatory conduction significantly speeds up the transmission of nerve impulses.