Final answer:
The correct answer to the student's question about which of the options decreases heart rate is D) increase in parasympathetic activity. Parasympathetic stimulation slows the heart rate through the release of acetylcholine.
Step-by-step explanation:
The student's question asks which of the following decreases heart rate. The options provided are related to various physiological responses that can affect the heart rate. Among them, the increase in parasympathetic activity is the one that leads to a decrease in heart rate. Parasympathetic stimulation, primarily via the vagus nerve, releases acetylcholine (ACh), which has the effect of hyperpolarizing the pacemaker cells in the heart, thus slowing the heart rate. In contrast, sympathetic stimulation increases heart rate and force of contraction. Additionally, increasing sympathetic activity, increasing venous pressure, and increasing stroke volume would all increase heart rate, while increasing filling time might have a variable impact on heart rate.
Therefore, the correct answer that causes a decrease in heart rate is D) increase in parasympathetic activity.