The therapist is arguing that a form of therapy that focuses on conscious beliefs (CB) is more likely to be successful than one focusing on unconscious ones (UB).
B and D are similar, but one is better than the other.
B says it is difficult for a therapy to work without focusing on CB.
D says a therapy focusing on UB cannot be effective if it does not address CB.
Clearly D is a stronger statement. B only means it is difficult for a therapy that focuses on UB to work, D means it is impossible.
Also,
the stimulus has one premise and a 'since premise2, thus conclusion' phrase, made more obtuse as 'thus...., since...'.
there is a logic gap between the second premise and the conclusion, (B) provides another premise to fill this gap.