IMO - A

attributed to - idiom

I will prefer A over D because the second clause describes the some consequences/insights by the action done in first clause and thus it is not a generic statement which the author i trying to make in the second clause and thus it is not a if-else thingy!

Despite the warnings that tobacco companies are now required to issue, some attorneys still attempt to file lawsuits against tobacco companies on behalf of smokers; but in attributing smoking to tobacco companies, the smokers are in effect told that they are not responsible for their own decisions

A. in attributing smoking to tobacco companies
B. in attributing the act of smoking to tobacco companies ("the act of smoking" is unnecessarily wordy)
C. in attributing tobacco companies as the cause of smoking ("Attributing as" is unidiomatic)
D. if smoking is attributed to tobacco companies (matches the passive voice of the next clause: "smokers are told")
E. if tobacco companies are attributed as the cause of smoking ("Attributing as" is unidiomatic)

We need "if" here; the consequence of "smokers being told..." depends on that condition being met.