What class of antiarrhythmic drugs blocks potassium channels prolonging phase 3 of the action potential and slowing the rate and conduction of the heart?

The correct answer is C: Class III. Class III antiarrhythmic drugs block potassium channels, which prolongs phase 3 of the action potential by delaying repolarization. This action results in a longer refractory period, slowing the heart rate and conduction. Option A (Class I) blocks sodium channels, option B (Class IV) blocks calcium channels, and option D (Class II) are beta-blockers that work by blocking beta-adrenergic receptors. These classes do not have a direct effect on potassium channels and do not produce the same effect as Class III drugs.