Final answer:
The motor neuron is the part of the reflex arc that connects the effector, typically a muscle, to the spinal cord, allowing for quick, involuntary responses like withdrawing a hand from a hot stove.
Step-by-step explanation:
The part of the reflex arc that connects the effector to the spinal cord is B. The motor neuron. In the reflex arc, a sensory neuron conveys information from the sensory receptors to the spinal cord, where an interneuron generally mediates the connection. Following this integration, a motor neuron carries the impulse from the spinal cord to the effector, often a muscle, triggering a response such as muscle contraction.
This sequence of events facilitates a swift reflex action without the need for conscious brain activity. For instance, when you touch a hot stove, almost instantly you withdraw your hand. This quick response is due to the reflex arc's efficiency, where the motor neuron serves as the critical link to the effector, bypassing the need for cognitive processing and enabling an immediate reaction.