Yes, it could be studied experimentally. But that is not what the question is asking, it is asking which would be least likely to be used.
If you were to do an experimental study using classical conditioning here, it would probably entail inducing physiological arousal in the patients, and measuring their "level" of panic. It is unethical to do that. You don't see P/S research using experimental conditions to exacerbate pt. symptoms.
You could definitely do a longitudinal study, and have them report on their levels of arousal/panic attacks.