Answer:
b. myelin
Step-by-step explanation:
Most motor neuron axons are myelinated, meaning they are covered by a myelin sheath, which is a “fatty” substance that isolates and protects the neuron's cell membrane. In the peripheral nervous system, this myelin sheath is made up of specialized cells called Schwann cells.
This sheath is not continuous, ie it does not surround the entire axon membrane; These uncovered spaces are known as Ranvier nodes.