Final Answer:
The portion of a reflex arc located entirely within the central nervous system is Interneuron. Thus, the correct answer is c) Interneuron.
Step-by-step explanation:
A reflex arc is a neural pathway that mediates a reflex action, involving a rapid and involuntary response to a stimulus. It comprises several components, including a sensory neuron, interneuron, and motor neuron. In this context, the interneuron is the portion of the reflex arc that resides entirely within the central nervous system (CNS).
Upon sensing a stimulus, the sensory neuron transmits the signal to the spinal cord or brainstem, where the interneuron comes into play. The interneuron acts as an intermediary, receiving and processing the sensory input before relaying signals to the motor neuron.
This interneuronal communication and processing occur entirely within the CNS, allowing for the integration and coordination of signals before initiating a motor response.
The motor neuron, responsible for carrying the response signal from the CNS to the effector organ (such as a muscle or gland), extends outside the CNS, completing the reflex arc. Once the interneuron processes the sensory information and decides on an appropriate response, it communicates with the motor neuron, which carries the instructions to the effector organ, eliciting the reflexive action.
Therefore, while the sensory and motor neurons partially reside outside the CNS, the interneuron functions solely within the CNS in the reflex arc.
Thus, the correct answer is c) Interneuron.