Answer : 4. "Neurons"
Explanation :

Answer: D) Neurons Explanation: The basic unit of the nervous system is the Neurons.   The neuron is a single nerve cell, and you have billions of them. There are three basic types of neurons:: 1. Association, 2. Afferent  and 3. Efferent. The association neurons comprise the central nervous system (CNS) that is the brain and spinal cord. The afferent neurons are also known as the sensory neurons, they bring the stimuli from the sensors (e.g., skin, eyes, ears) to the CNS. The efferent neurons are also known as motor neurons, they bring the responses from the brain to the muscles and the glands.