**Core Concept**
The question is testing the student's understanding of the pharmacological effects of cholinergic drugs on the heart, specifically how they decrease the levels of cyclic adenosine monophosphate (cAMP) and open potassium (K+) channels, leading to various cardiac effects.

**Why the Correct Answer is Right**
The correct answer is a muscarinic receptor agonist that activates the Gi protein-coupled pathway, which inhibits adenylate cyclase and decreases the levels of cAMP. This decrease in cAMP levels leads to the opening of K+ channels, resulting in hyperpolarization of the cardiac cell membrane, which in turn decreases the heart rate. The opening of K+ channels is mediated by the activation of the muscarinic receptor subtype M2, which is primarily found in the heart.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because beta-2 adrenergic receptor agonists increase the levels of cAMP, which would have the opposite effect of increasing the heart rate.
**Option B:** This is incorrect because calcium channel blockers decrease the influx of calcium ions into the cardiac muscle cells, which would not be related to the opening of K+ channels.
**Option C:** This is incorrect because this class of drugs is primarily used for their anti-arrhythmic properties and works by blocking sodium channels, which would not fit the mechanism described in the question.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the muscarinic receptor subtypes M2 and M3 have different effects on the heart. M2 receptors are primarily involved in the regulation of heart rate, while M3 receptors are involved in the contraction of smooth muscle in the gastrointestinal tract.

**Correct Answer: C. Acetylcholine**