We don't know if the debate is exciting. Also, the people most likely to watch it are already committed voters. But even if it is exciting and causes more people to vote, (A) doesn't address the conclusion that winning it doesn't improve one’s chances of winning the election.

The author only addresses how televised debates affect viewers, saying that many are already committed to one candidate and the rest are unsure who won. But he fails to address how televised debates affect non-viewers. Their votes might be influenced by reports about the debate.

The premises state that many viewers are already committed to one candidate or another; it makes sense that they’d be likely to disagree about who won the debate. But this doesn’t affect the conclusion that winning doesn’t impact one’s chance of winning the election.

Whether voting behavior is influenced in “unpredictable ways” by a debate doesn’t affect the conclusion that winning a debate doesn’t impact one’s chances of winning the election. Are these “unpredictable ways” positive or negative for the winner? We just don’t know.

The author doesn’t fail to consider this. He explicitly says that, because many viewers are committed to a particular candidate, winning or not winning a debate doesn’t affect one’s chance of winning an election.