The simplest response to a stimulus is a?

a.sympathetic
b.voluntary movement
c. reflex

The correct answer is c. reflex. A reflex is the simplest and most basic type of response to a stimulus. It is an involuntary and almost instantaneous reaction that occurs without conscious thought. Reflexes are controlled by the nervous system and serve to protect the body from potential harm. They are typically mediated by specific nerve pathways called reflex arcs.

To arrive at this answer, you can apply your knowledge of human physiology and the different types of responses to stimuli. Understand that sympathetic responses are typically associated with the autonomic nervous system's fight-or-flight response, which is more complex than a simple reflex. Voluntary movements, on the other hand, require conscious thought and decision-making processes, making them more complex than a reflex. Therefore, the simplest response to a stimulus would be a reflex.