Final answer:
Upon the arrival of a second trained responder with an AED, use the AED on the adult while continuing CPR until the AED advises whether or not a shock is necessary.
Step-by-step explanation:
When performing CPR on an adult and a 2nd trained responder arrives with an AED (Automated External Defibrillator), the correct action would be to use the AED on the adult. AEDs are designed to be used by both trained and lay persons and can automatically diagnose the patient's heart condition, applying a shock with appropriate energy and waveform. While CPR is crucial for maintaining blood flow until the heart resumes beating, the arrival of an AED presents an opportunity to potentially rectify dangerous arrhythmias such as ventricular fibrillation by delivering a controlled electrical shock to the heart. In such emergency situations, the use of an AED should complement CPR efforts, not replace them until it's determined that a shock is necessary or advised by the AED's automated system.