Sinus arrhythmia is due to-

Answer-C. Fluctuating parasympathetic response during respirationSinus rhythm with a beat-to-beat variation in the P-P interval (the time between successive P waves), producing an irregular ventricular rate.Sinus arrhythmia is a normal physiological phenomenon, most commnonly seen in young, healthy people.The hea rate varies due to reflex changes in vagal ( parasyntpathetic ) tone during the different stages of the respiratory cycle,Inspiration increases the hea rate by decreasing vagal tone.With the onset of expiration, vagal tone is restored, leading to a subsequent decrease in hea rate.