Which structure contains both sensory and motor nerve fibers in the reflex arc?

The correct answer is the spinal nerve because it is the structure that contains both sensory and motor nerve fibers, which are essential for completing the reflex arc. In the reflex arc, sensory neurons transmit signals from receptors to the spinal cord, while motor neurons carry signals from the spinal cord to effectors, such as muscles. This combination of sensory and motor pathways enables the body to respond quickly to stimuli without the need for conscious brain involvement.

Other options, such as the medulla oblongata, brainstem, and cerebellum, do not contain a complete pathway with both sensory and motor fibers in the same structure as the spinal nerve. While the brainstem and medulla oblongata are involved in processing reflex actions and motor control, they do not directly serve as the main path for both sets of fibers like the spinal nerve does. The cerebellum primarily plays a role in coordination and balance but does not directly contribute to the reflex arc in terms of carrying sensory and motor signals.