The correct answer is D) by applying his mouth tightly over the victim's mouth. This is the correct answer because it provides the best airtight seal during mouth-to-mouth ventilation. By placing the rescuer's mouth directly over the victim's mouth, it creates a tight seal that allows for effective air exchange and delivery of breaths to the victim. This technique is the standard recommended approach for providing rescue breaths during CPR. The other options are incorrect because they do not create a proper airtight seal. Cupping a hand around the mouth (A) or holding the jaw down (B) may help position the airway, but do not seal the mouth. Keeping the head elevated (C) is important for airway positioning but does not directly address creating the airtight seal.