The correct answer is diltiazine. The observation is that the action potential prolongation observed in A with these early after depolarizations, these interruptions in the normal repolarization of the action potential are eliminated by the intervention. These are typically due to reactivation of depolarizing currents, most often at this voltage, L-type calcium currents. So it's not surprising that a calcium channel blocker like diltiazine will not only block these early after depolarizations, but will also shorten the action potential duration. Dronetarone will tend to prolong the action potential duration, and flecainide and Esmolol may not have consistent effects like diltiazine would to produce this action potential shortening and elimination of EADs.

diltiazine

calcium channel blocker

early after depolarizations

action potential

L-type calcium currents