Final answer:
When an AED is available during CPR, the first step is to turn on the AED immediately for potential use in restoring the heart's normal rhythm.
Step-by-step explanation:
When an AED (Automated External Defibrillator) becomes available during a one-rescuer CPR situation, the first action you should take is option D: Turn the AED on. As soon as the AED is available, it should be integrated into the resuscitation efforts without delay. While chest compressions are a critical part of CPR, the prompt use of an AED can be a lifesaver as it is designed to diagnose the heart's condition and apply a shock if necessary to restore the heart's normal rhythm, especially in the case of ventricular fibrillation or pulseless ventricular tachycardia, which are common causes of cardiac arrest.
Answer: D. Turn the AED on.