**Core Concept**
The autonomic nervous system regulates heart rate through the action of neurotransmitters such as acetylcholine (ACh) and adrenaline. ACh, released from the vagus nerve, acts on the sinoatrial (SA) node to decrease heart rate.

**Why the Correct Answer is Right**
Acetylcholine decreases heart rate by stimulating the muscarinic receptors (M2 subtype) on the SA node, which leads to an increase in potassium conductance and a decrease in calcium conductance. This results in a decrease in the rate of depolarization of the SA node, thereby slowing down the heart rate. The activation of M2 receptors also inhibits the L-type calcium channels, further contributing to the decrease in heart rate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the action of acetylcholine on heart rate.
**Option B:** While beta blockers do decrease heart rate, they do so through a different mechanism, namely by blocking the action of adrenaline on beta-1 receptors.
**Option C:** This option is incorrect as acetylcholine does not increase heart rate; it decreases it.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the autonomic nervous system has a dual effect on the heart, with the parasympathetic nervous system (ACh) decreasing heart rate and the sympathetic nervous system (adrenaline) increasing heart rate.

**Correct Answer: A. Stimulation of muscarinic receptors on the SA node.**