After using an AED on a patient lying on his back with a pulse and adequate breathing, what is his position?

The correct choice is supine, which describes a patient lying on their back, facing upward. In this scenario, using an Automated External Defibrillator (AED) involves placing pads on the patient's chest to analyze and treat any potentially life-threatening arrhythmias, which is typically done while the patient is on their back for accessibility and accuracy in delivering therapy.

Being in a supine position is important because it ensures that the AED equipment can make proper contact with the patient's chest without any interference from clothing or other obstacles. This position also allows for easy monitoring of the patient’s vital signs, including pulse and breathing, which remain stable in this case.

The other positions listed do not apply here: a prone position would mean the patient is lying face down, which would make it impractical for AED use; sitting would be unsafe for a patient in need of electrical therapy, as it may not provide the stable surface needed for effective AED placement; and Fowler's position, where the patient sits at an angle, is not conducive for AED use either, as it similarly complicates the necessary contact for the shock pads. Thus, supine is indeed the appropriate and necessary position for this scenario.