Final answer:
An atrioventricular block is the correct answer as it specifically affects the PR interval with degrees of conduction delay or interruption between the atria and ventricles.
Step-by-step explanation:
A possible variation cause with PR interval is b) Atrioventricular block. This condition involves a delay or interruption in the electrical signals passing from the atria to the ventricles. An atrioventricular (AV) block is often described by degrees; first-degree or partial block indicates a delay between the SA and AV nodes, which is recognized on the ECG as an abnormally long PR interval. A second-degree or incomplete block occurs when some impulses are blocked, resulting in some P waves not followed by a QRS complex. In a third-degree or complete block, there is no correlation between P waves and QRS complexes, signifying a complete disruption in the conduction of electrical impulses from the atria to the ventricles. In contrast, conditions like atrial fibrillation, premature ventricular contraction, and sinus tachycardia affect other aspects of the ECG but do not primarily cause variation in the PR interval.