Acomplete pathway through the nervous system, beginning with a stimulus and ending with a response, is called a/n:
a. reflex arc
b. neurotransmitter
c. synaptic cleft
d. dermatome

A. reflex arc  

Explanation:

The reflex arc is the immediate response to the excitement of a nerve without the will or consciousness of the animal, ie it is a stimulus that does not reach the brain, it receives response in the marrow.

Reflex arcs can be simple or complex depending on the number of components involved. In the case of complex reflexes we have several muscles responding to a stimulus, such as swallowing and scratching.  In simple reflexes, the components are smaller and usually the response is obtained in a single effector organ, such as in the stretch reflex, where a single muscle is recruited.