Final answer:
The priority nursing diagnosis is Altered Nutrition, less than body requirements.
Step-by-step explanation:
The priority nursing diagnosis in this case is C) Altered nutrition, less than body requirements. The teenage female client who is admitted to the psychiatric hospital for possible anorexia nervosa is at high risk for altered nutrition due to her low weight and the extreme caloric restriction associated with the eating disorder. This diagnosis takes priority because without addressing and treating the malnutrition, the client's health will continue to deteriorate, leading to serious complications.