(D) is clearly correct, but I'm having trouble articulating why (C) doesn't work. Here is my articulation:
At its core I think it comes down to a necessary vs. sufficient concept. Not like "C is sufficient but not necessary" but like this: the stimulus is saying that in order to help a patient heal, the psychotherapist MUST focus on positive change in relationships. The negation of (C) would be "there are at least some patients who will not find relief by changing their relationships." BUT the stimulus wasn't saying "if the psychotherapist focuses on positive change, the patient will be helped," it was saying "focusing on positive change is necessary in order to help." So it doesn't kill the argument to say "there are some cases where positive change didn't help a patient."
Maybe that isn't as muddled as I thought but any input would be helpful. Thanks!