Which emergency situation requires the use of an AED?

The situation that requires the use of an Automated External Defibrillator (AED) is when a person is in cardiac arrest, which is indicated by being unresponsive and not breathing normally. In this critical condition, the heart has stopped functioning effectively, and immediate intervention is necessary to restore a proper heart rhythm. An AED is designed to analyze the heart's rhythm and deliver a shock if needed, potentially saving the person's life.

In contrast, severe allergic reactions, fainting, and heat exhaustion do not typically involve the immediate need for an AED. Severe allergic reactions, while serious, usually require the administration of epinephrine and medical assistance rather than defibrillation. Fainting may not involve cardiac issues; instead, it is often due to temporary drops in blood flow to the brain and does not necessitate the use of an AED. Heat exhaustion, while also serious, is generally treated by cooling the body and rehydrating, without involving cardiac arrest issues. Thus, option B specifically addresses the life-threatening scenario for which an AED is crucial.