We have to find a
reason why people who succeed in breaking their addiction to smoking cigarettes are more likely to be motivated by the social pressure against smoking than by health concerns.
(A) a habit that involves an addictive substance is likely to pose a greater health threat than a habit that does not involve any addictive substance
Incorrect, comparing what happens in an addictive substance with happens in non-addictive substance is of no help for us to understand why people choose social pressure over health concerns in the additive substance quitting.
(B) for most people who successfully quit smoking, smoking does not create an immediate health concern at the time they quit
Correct, gives us a reason that people don't see much of a health concern when quitting smoking so they don't think about it and choose social pressure instead.
(C) some courses of action that exacerbate health concerns can also relieve social pressure
Incorrect, talks about some courses of action and doesn't talk specifically about smoking which is relevant to us.
(D) most people who succeed in quitting smoking succeed only after several attempts
Incorrect, doesn't explain why people choose social pressure over immediate health concerns.
(E) everyone who succeeds in quitting smoking is motivated either by social pressure or by health concerns
Incorrect, goes against the conclusion, if people are motivated by both then it contradicts the conclusion and confuses us why people choose social pressure over immediate health concerns.