D - The human nervous system includes billions of individual neurons that assist with internal communication.
An estimate is that the human brain contains around 80 billions individual neurons with A LOT more synapses - the points of contact where information gets transmitted. Neurons are responsible for our internal communication. Every motoric move you make, every sensory stimuli your receive through your eyes, ears, or through your tongue, every thought you make, everything is a consequence of an elaborate dance of continuously active neurons and the different connections between them that get activated again and again.