The correct answer is B) applying his mouth tightly over the victim's mouth. To provide an airtight seal during mouth-to-mouth resuscitation, the rescuer needs to cover the victim's mouth completely with their own mouth. This creates a tight seal that allows the rescuer's breath to effectively enter the victim's lungs. Pinching the victim's nostrils closed helps prevent air from escaping and ensures the air is directed into the lungs. The other options are incorrect because they do not create the necessary airtight seal. Keeping the head elevated (A) or cupping a hand around the mouth (C) do not properly seal the airway. Holding the jaw down firmly (D) may help open the airway, but does not create the airtight seal required for effective mouth-to-mouth resuscitation.