Which of the following conditions require immediate defibrillation?

Ventricular fibrillation is a critical condition that requires immediate defibrillation because it represents a disorganized electrical activity in the heart that prevents it from effectively pumping blood. In this state, the heart's quivering muscles cannot produce a pulse, which leads to rapid deterioration and can result in death if not addressed promptly. Defibrillation aims to deliver a high-energy shock to the heart, allowing it to reset and ideally restore a normal rhythm.

Other conditions listed, such as asystole, stable atrial fibrillation, and symptomatic bradycardia, do not require immediate defibrillation. Asystole indicates a complete lack of electrical activity in the heart and is not responsive to defibrillation. Stable atrial fibrillation generally does not require emergency intervention, as the patient may be stable and can be managed with medications. Symptomatic bradycardia may require interventions like atropine or pacing but not defibrillation. Thus, the urgency and necessity of defibrillation in the case of ventricular fibrillation make it the correct answer.