Which of the following is an unshockable rhythm?

Asystole is classified as an unshockable rhythm because it indicates a complete absence of electrical activity in the heart, resulting in no detectable pulse. In the context of advanced cardiac life support protocols, the focus for treatment of asystole involves addressing underlying causes and implementing effective cardiopulmonary resuscitation (CPR), rather than delivering a shock, which is ineffective in this situation.

Other rhythms listed, such as ventricular tachycardia and monomorphic ventricular tachycardia, exhibit organized electrical activity that may lead to effective contraction of the heart. Atrial fibrillation, while not a normal rhythm, can still have some electrical activity that may respond to defibrillation. Therefore, asystole specifically is recognized as a rhythm where electrical shocks are not indicated, making it an unshockable rhythm.