Final answer:
The brain is not part of a simple reflex; it involves sensory neurons, motor neurons, and sometimes interneurons at the spinal cord level for rapid response.
Step-by-step explanation:
The part that is not part of a simple reflex is c. brain. A simple reflex arc typically includes a sensory neuron, which detects the stimulus, and a motor neuron, which carries the signal to the muscle for a response. Often, there is also an interneuron within the spinal cord that links the sensory and motor neurons. The brain is not directly involved in a simple reflex arc as the reflex action occurs at the level of the spinal cord, allowing for a quicker response.