Hi, the author argues that their form of therapy is better, since it focuses on things in peoples direct control. While this argument feels decent, we don't know that focusing on things in peoples direct control is actually better. B gives us that, which is why b is right.
D tells us that focusing on unconsciousness beliefs can't be effective without also focusing on conscious ones. While this might attack the idea that it is good to focus on unconscious beliefs it doesn't actually establish that it is better to focus on conscious ones, rather, it only establishes that one is necessary for the other.