An ECG consists of a set of three waves; these are namely the P wave, a QRS complex, and a T wave. Initiation of impulse by SA nodes and its travel through atria is represented by P wave which in turn signals systole of atria and initiation of contraction of atrial myocardium. This makes option A correct answer. Depolarization of the ventricles followed by excitation of the Purkinje fibers is represented by QRS complex which makes option B incorrect answer. It signals ventricular systole and initiation of contraction of ventricular myocardium. Repolarization of the ventricles is represented by T wave which makes option D incorrect. It signals ventricular diastole and relaxation of ventricular myocardium. The periods from initiation of QRS complex to peak of T wave is referred to as absolute refractory period when cardiac cells do not respond to any stimulus irrespective of it strength. This makes option C incorrect. Correct answer is A.