The part of the nervous system involved in a reflex action is the:

The spinal cord is a crucial component of the nervous system that plays a fundamental role in the mediation of reflex actions. When a stimulus is detected by a receptor, the sensory information is transmitted via the sensory neuron to the spinal cord, where it is processed. The sensory information is interpreted appropriate motor response is then generated.

Hence, the correct option is the spinal cord.