The television network's advertisement for its new medical drama gross

Premise:
Network's ad misrepresents the program.

Conclusion:
It will not as effectively attract the sort of viewers likely to continue watching the program as would the advertisement that the program's producers favored.

Note that the conclusion concludes not just "the network's ad will not be effective in attracting suitable viewership" (which is valid), but it also concludes that the producer's ad would have been more effective. We have no reason to believe that.

The assumption is that the producer's ad does not misrepresent. This is given in option (B) so that is correct. Let's look at the other options too.

A. Most viewers who tune in to the first episode of the program will do so because of the network's advertisement for the program.

The argument talks about people who tune in because of the network's ad. What percentage they represent out of total viewership is outside the scope of the argument.

C. Most people who tune in to the first episode of the program and become loyal viewers will not have tuned in to the first episode as a result of the network's advertisement for the program.

Same logic as (A)

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.

No. We are only concerned about people who get attracted by Ads. There could be a lot of people who would watch the program after reading an article about it in the news that such a program is being launched. There could be many viewers who watch anything that comes in that time slot... whatever. Point is that almost all viewers who tune in to the first episode may not have been the Ad watchers. So the author is not assuming that if producer's ad would have been used, almost all first time viewers would have stuck around. He is assuming that producer's ad would have been more effective in attracting (more) regular viewers.

E. Most people who become loyal viewers of a program do not miss the program's first episode.
Out of scope. What kind of viewers watch the first episode has no impact on our argument.

Answer (B)