Final answer:
The parasympathetic nervous system reduces heart rate through stimulation of the vagus nerve, which releases acetylcholine and slows the rate of depolarization of cardiac cells.
Step-by-step explanation:
The parasympathetic nervous system acts to reduce heart rate by the stimulation of the vagus nerve, which is answer (d) to the student's question. When the parasympathetic nervous system is activated, acetylcholine (ACh) is released from the vagus nerve at the neuromuscular junctions. This neurotransmitter acts on the SA and AV nodes, opening potassium channels that lead to hyperpolarization of cardiac cells, slowing the rate of spontaneous depolarization. This means that it takes longer for the cells to reach the threshold for generating a heartbeat, effectively slowing down the heart rate.