A. the more difficult it is for the addict to quit as to do so requires a reorganization of the neuro-biological mechanisms responsible for compulsive behavioral patterns...........................Looks fine
B, the more difficult it is for the addict to quit as
it requires a reorganization of the neuro-biological mechanisms responsible for compulsive behavioral patterns...........................Antecedant of 'it' is not clear. It should refer to noun but in the context of the sentence we require something that refers to 'quitting'.
C.
it is more difficult for the addict to quit as doing so requires a reorganization of the neuro-biological mechanisms responsible for compulsive behavioral patterns.....................................
The longer.......the more is correct construction
D.
difficulty increases for the addict to quit as to do so requires a reorganization of the neuro-biological mechanisms responsible for compulsive behavioral patterns.................................Same as C
E. the more difficult it is for the addict to quit as to do so requires a reorganization of the neuro-biological mechanisms
that are responsible for the patterns of compulsive behavior...........................wordy sentence.
IMO OA should be A