**Core Concept**
The calculation of heart rate from an electrocardiogram (ECG) is essential for diagnosing and managing various cardiac conditions. It involves measuring the time interval between two consecutive R waves in the ECG tracing.

**Why the Correct Answer is Right**
To calculate heart rate from an ECG, the correct formula is: Heart Rate = 60,000 / (RR interval in milliseconds). This formula is based on the fact that there are 60,000 milliseconds in one minute, and the RR interval represents the time between two consecutive R waves. This calculation can be used to determine the heart rate in beats per minute (bpm). The RR interval is measured from the peak of one R wave to the peak of the next R wave.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide a valid formula for calculating heart rate from an ECG. The formula provided is not accurate and would not yield a reliable result.

**Option B:** This option is incorrect because it does not specify a valid formula for calculating heart rate from an ECG. The mention of "ECG cycle" is vague and does not provide a clear method for determining heart rate.

**Option C:** This option is incorrect because it does not provide a valid formula for calculating heart rate from an ECG. The mention of "cardiac cycle" is related to the cycle of electrical activity in the heart, but it does not provide a clear method for determining heart rate.

**Clinical Pearl / High-Yield Fact**
When calculating heart rate from an ECG, it's essential to measure the RR interval from two consecutive R waves and use the correct formula: Heart Rate = 60,000 / (RR interval in milliseconds). This ensures accurate calculation of heart rate, which is critical for diagnosing and managing various cardiac conditions.

**Correct Answer:** C.