Which of the following is a better predictor of vagal tone

In the given options, all are influenced by sympathetic and parasympathetic activity. Right vagus mainly innervates SA node, right atrium and left vagus innervates left atrium, AV node, a bundle of His. But vagal innervation of the ventricle is sparse. Hence, vagal stimulation strongly inhibits heart rate (strong - negative chronotropic effect) and poorly affects the force of contraction (weak - negative inotropic effect). Moreover, vagus dominates sympathetic nerve in controlling SA node activity at rest. Therefore, basal heart rate mainly depends on the vagal tone. Ejection fraction, stroke volume, LVET (Left Ventricular Ejection Time) are mainly influenced by sympathetic stimulation as the vagus has a weaker inotropic effect.