**Core Concept**
The autonomic nervous system, specifically the vagus nerve, plays a crucial role in regulating various bodily functions, including heart rate, digestion, and respiration. Vagal tone refers to the overall activity of the vagus nerve, which can be influenced by various factors, including genetics, environment, and lifestyle.

**Why the Correct Answer is Right**
High-frequency heart rate variability (HF-HRV) is a measure of the variation in the time interval between heartbeats, which is influenced by the vagus nerve. Studies have shown that HF-HRV is a reliable indicator of vagal tone, as it reflects the parasympathetic nervous system's ability to regulate heart rate. HF-HRV is measured using techniques such as electrocardiography (ECG) or pulse oximetry. The parasympathetic nervous system, mediated by the vagus nerve, releases acetylcholine, which binds to muscarinic receptors on the sinoatrial node, causing a decrease in heart rate.

**Why Each Wrong Option is Incorrect**
* **Option A:** Low-frequency heart rate variability (LF-HRV) is not as reliable a measure of vagal tone as HF-HRV, as it is influenced by the sympathetic nervous system as well.
* **Option B:** The respiratory rate is not a direct measure of vagal tone, although it can be influenced by the autonomic nervous system.
* **Option D:** The coefficient of variation (CV) of R-R intervals is a measure of variability, but it does not specifically reflect vagal tone.

**Clinical Pearl / High-Yield Fact**
HF-HRV is a valuable predictor of cardiovascular disease and mortality, and has been shown to be a marker of resilience and adaptability in the face of stress.

**Correct Answer:** C. High-frequency heart rate variability (HF-HRV).