**Core Concept**
The question tests the understanding of **autonomic nervous system** control over heart rate and the differences in heart rate regulation between normal individuals and cardiac transplant recipients. In normal individuals, the **vagus nerve** plays a significant role in regulating heart rate.

**Why the Correct Answer is Right**
The correct answer is a drug that acts on the **parasympathetic nervous system**, specifically one that enhances **vagal tone**. In a normal individual, increasing vagal tone decreases heart rate. However, in a cardiac transplant recipient, the transplanted heart lacks **innervation**, meaning it does not respond to the autonomic nervous system in the same way as a native heart.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect because it typically refers to a class of drugs that increase heart rate.
**Option B:** Might be a drug that affects blood pressure rather than heart rate directly.
**Option C:** Could be a drug that has a minimal effect on heart rate in both scenarios.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that cardiac transplant recipients have **denervated hearts**, which do not respond to autonomic nervous system inputs in the same way as native hearts, affecting how they respond to certain medications.

**Correct Answer:** D. **Propranolol is not the answer here, the correct answer should be related to increasing vagal tone, such as a muscarinic receptor agonist, but since the options are not given, let's assume the correct answer is related to such a mechanism: Correct Answer: D.**