(D) "If the advertisement that the program's producers favored were used instead of the network's advertisement, almost all of the viewers who tuned in to the first episode would tune in to subsequent episodes as well."
This is incorrect because of the phrase "almost all." The argument does not assume that most viewers would tune in again if they saw the producers' favored ad, just that more viewers would tune in than would under the other ad. In other words, (D) is too strong of a claim for the argument to rely on. The passage only implies that the other ad will "not as effectively attract...viewers" as the producers' favored ad would. We are told that the other ad attracts people who "will be unlikely to watch subsequent episodes," so the producers' favored ad must attract people who are more likely to watch subsequent episodes, but this does not necessarily mean that "almost all" of the viewers have to watch subsequent episodes.
Does that make sense? Please let us know if you have any other questions!