How would a clinical psychology researcher know whether client-centered therapy

How would a clinical psychology researcher know whether client-centered therapy is effective in treating anxiety? Clients randomly assigned to be hypnotized report lower anxiety than those who are not hypnotized. A Clients test lower in anxiety after a reframing exercise than they did before the exercise. B Clients randomly assigned to be spoken to with unconditional positive regard have lower cortisol levels than those in a control group. C Clients who take benzodiazepines have lower heart rates than those who take a placebo. D Clients who engage in weekly group sessions report lower anxiety than those who have weekly solo sessions with a therapist.