Final answer:
The best method of a vagal maneuver for a 7-month-old patient with supraventricular tachycardia who is hemodynamically stable is the modified Valsalva maneuver.
Step-by-step explanation:
The best method of a vagal maneuver for a 7-month-old patient with supraventricular tachycardia who is hemodynamically stable is called the modified Valsalva maneuver. This maneuver involves having the patient blow into a syringe or adapter to create pressure in the chest, which in turn stimulates the vagus nerve and helps slow down the heart rate.