Final answer:
The proper method for measuring the appropriate length to use when inserting a nasopharyngeal airway is from the tragus of the ear to the nostril plus 1 inch.
Step-by-step explanation:
The proper method for measuring the appropriate length to use when inserting a nasopharyngeal airway is described in option a. When holding the airway on the side of the patient's face, it should reach from the tragus of the ear to the nostril plus 1 inch. This measurement ensures that the airway will effectively reach the back of the nasopharynx and maintain an open air passage.