Answer:D. myelin sheath.
Explanation:living organisms have the ability to respond to stimuli .in humans ,the nervous system is responsible for this function.the nervous system consists of the brain , the spinal cord and nerves.
Neurons are found in the nervous system and help in the propagation of impulses across the body.a neuron usually consists of a dendrite, cell body and axon.
The axon extends from the cell body to the dendrite.
The axon is surrounded by the myelin sheath,which forms a whitish,fatty layer around it.the myelin sheath is produced by Schwann cells also called neurilemma.the neurilemma produces the myelin sheath.
The neurilemma is interrupted at intervals by the node of ranvier.
These features helps impulse moves faster along the neuron by reducing the distance it travels.
The Schwann cells is also responsible for the maintainance and regeneration of neurons
