The passage states that because many people tuned their channels to the station which had started airing the half hour long speech, it can be concluded that people not only like small channels as is generally known, but also like long speeches, unlike what was believed earlier.
We have to find a gap or a scenario which can weaken this conclusion. In short, we have to try and prove that people actually don't like long speeches as is being concluded now. What if many people came to the channel without knowing what was being played at that channel at that moment, and by chance bumped into this long speech. Also, what if people sure came to the channel knowingly or unknowingly, but did not stay long enough so as to help us conclude that they like long speeches.
Looking at options :
A) Well, the candidate can normally be judged on many other parameters as well. May be people liked his speech but not his personality or actions on the ground level. May be people just listened to him so as to know what is going on in the minds of politicians nowadays. This option does not have a strong link with the conclusion. It should be an incorrect choice.
B) If the ad was advertised for very long, it makes one of my weaken points nullified. It actually means that people did not just unknowingly bumped into the ad. They knew very well what it will contain and for how long it will last.It shows that they came to listen with interest. If they had no interest, they would never have come in the first place. This option acts as a strengthener. Incorrect choice.
C) If this ad was aired during a time slot normally taken by a very famous show, it becomes likely that people wanted to see that show, but accidentally happened to be present at the beginning of this long ad. In such a case, its difficult to conclude that they came because they were interested. This option acts a weaken statement and is correct.
D) This option talks about how the small ads are shown to the public, but does not talk about the long ones. It seems to have no link and is only trying to divert our attention to short length ads. Incorrect choice.
E) So a regular length program is famous if the moderator has good qualities. This option hints that the moderator of this long ad was in fact liked by the public, so people came in large numbers. This adds weight to the conclusion and is definitely not the correct choice.
Answer choice should be C