The people most likely to watch a televised debate between political

One of those questions in which focusing on the conclusion is extremely important (I admit it is important in most questions!)

Premises:
People most likely to watch the debate are those who have already made up their mind (so candidate wins/loses is irrelevant to them)
Uncommitted viewers are generally undecided about who won the debate. So winning doesn't lead to these viewers voting for the winning candidate.

Conclusion: Winning a televised debate does little to bolster ones chances of winning an election.

We need to weaken it. The conclusion says winning debate doesn't do much to increase chances of winning the election. We need to show how "winning the debate could increase one's chances of winning the election". What is it that we failed to consider? What is it that shows that "winning the debate could increase one's chances of winning the election".

(A) watching an exciting debate makes people more likely to vote in an election

We are talking about the people who watched the debate. If they were committed, they already knew who to vote for. If they were uncommitted, they couldn't figure out who won. Hence, even if more of them vote, will they vote for the winning candidate, we can't say. Either they were already committed or couldn't make out who won. So winning doesn't help.

(B) the voting behavior of people who do not watch a televised debate is influenced by reports about the debate.

Ok, so there are reports about the debate and win/loss. The voting behaviour of people who do not watch the debate could be influenced by publishing of a win. Hence, winning could help tilt the scale in the winner's favour. So winning could increase chances of winning the election. Correct.

(C) there are differences of opinion about what constitutes winning or losing a debate

Whether winning is tightly defined is irrelevant. Anyway, the argument says winning doesn't help. So this option doesn't say that winning helps.

(D) peoples voting behavior may be influenced in unpredictable ways by comments made by the participants in a televised debate

Here is the trick in this option - unpredictable ways by comments. This option says that comments (whether winner makes or loser is irrelevant) could make people behave in an unpredictable manner (whether they vote for winner or not, we cannot say). Hence, it doesn't say that "winning the debate" could help in any way.

(E) people who are committed to a particular candidate will vote even if their candidate is perceived as having lost a televised debate
This essentially says what the argument says. Hence it doesn't weaken the argument.

Answer (B)