The right answer is spinal cord.
When a stimulus, calling for an immediate response, is perceived by a receptor housed in a part of the body, the nerve impulse propagates on the afferent fibers towards the spinal cord or the brainstem. After the passage through one or more synapses of the motor neurons, the afferent response - in the form of a second nerve impulse - goes to the effector organ (muscle for example) or to the motor centers, upper or lower (medullary nuclei of spinal cord or central nuclei, muscle tone of the cerebellum, awareness of the cortex, etc.)
Reflexes are sometimes stimulated for diagnostic purposes, especially in the neurological assessment; each reflex corresponds to an area of the spinal cord.