Final answer:
The PR interval represents the time from the beginning of atrial contraction to the beginning of ventricular contraction.
Step-by-step explanation:
The correct answer is b. PR interval. The PR interval measures the time from the beginning of atrial contraction (represented by the P wave) to the beginning of ventricular contraction (represented by the QRS complex). It is clinically relevant as it reflects the time it takes for the electrical signal to travel from the atria to the ventricles. The PR interval can provide important information about the conduction system of the heart.