the ECG showing different p-wave morphologies and the irregular interval between p-waves are suggestive of multifocal atrial tachycardia (MAT).
Treatment of underlying disease is the mainstay of the management of MAT. If patients are symptomatic treatment should begin with the correction of hypokalemia to maintain >4 mEq/L and hypomagnesemia to maintain >2 mEq/L.
In the absence of pulmonary disease, beta-blockers are the first-line treatment. In the presence of pulmonary disease, a non-dihydropyridine calcium channel blocker such as verapamil or diltiazem is the first line. Ablation of the AV node is usually done when patients do not respond to or cannot tolerate pharmacologic therapy. Electric cardioversion is ineffective.
Since the question asks which modality is ineffective in the management of this condition, i.e., MAT (and not specifically this patient), the answer would be B.
Thank you.