Answer:
A. Neurons; nerves
Explanation:
The nervous system is comprised of two types of a system called the peripheral and the central nervous system.
A neuron is the functional unit of the nervous system which transmits the signals to the organs and to the brain.
The central system is composed of the neurons of the spinal cord and the brain whereas the peripheral system is composed of the neurons which connect the central nervous system to the organs, therefore, the neurons of the peripheral system are the considered the extensions of the central nervous system and thus are called nerves composed of axons of the neurons.
Thus, Option-A is the correct answer.
