**Core Concept**
The vagus nerve, a branch of the parasympathetic nervous system, plays a crucial role in regulating heart rate and rhythm through the release of acetylcholine, a neurotransmitter that activates muscarinic receptors in the heart.

**Why the Correct Answer is Right**
When the vagus nerve is stimulated, it releases acetylcholine, which binds to muscarinic receptors on the sinoatrial (SA) node and atrioventricular (AV) node. This binding action slows the firing rate of the SA node and prolongs the refractory period of the AV node, resulting in a decrease in heart rate. The increased time between heartbeats is reflected as an increased RR interval on an electrocardiogram (ECG). The slowing of the heart rate is a classic example of the parasympathetic nervous system's influence on the cardiovascular system.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased heart rate is the opposite effect of vagal stimulation, which is mediated by the parasympathetic nervous system.
**Option C:** Cardiac output is the product of heart rate and stroke volume, and while vagal stimulation can decrease heart rate, it can also decrease contractility and increase peripheral resistance, leading to a net decrease in cardiac output.
**Option D:** The force of contraction is primarily influenced by the sympathetic nervous system through the release of adrenaline, not by vagal stimulation.

**Clinical Pearl / High-Yield Fact**
Vagal stimulation can also lead to a compensatory increase in stroke volume, a phenomenon known as the Frank-Starling mechanism. This highlights the complex interplay between the parasympathetic and sympathetic nervous systems in regulating cardiovascular function.

**✓ Correct Answer: B. Increased RR interval in ECG**