Final answer:
Increased efferent vagal nerve activity, which is an aspect of the parasympathetic nervous system, releases acetylcholine and leads to a decreased heart rate through the hyperpolarization of cardiac muscle cells.
Step-by-step explanation:
The original question posed by the student is "Which of the following would decrease heart rate?" Among the options provided, the one that decreases the heart rate is D) Increased efferent vagal nerve activity. The vagus nerve is a part of the parasympathetic nervous system, which when stimulated, releases acetylcholine (ACh) at the neuromuscular junction. This action results in a decrease in heart rate through the hyperpolarization of the cardiac muscle cell membrane and a reduction in the force of contractions. It directly counteracts the effects of the sympathetic nervous system, which is typically associated with an increase in heart rate due to the positive inotropic effect of sympathetic stimulation such as the release of norepinephrine (NE) and epinephrine (E).