My guess is B

The passage is arguing against the idea that the media is the dominant factor in elections. As evidence it is noting the inability of 3rd parties to get elected.

We have heard a good deal in recent years about the declining importance of the two major political parties. It is the mass media, we are told, that decide the outcome of elections, not the power of the parties. But it is worth noting that no independent or third-party candidate has won any important election in recent years, and in the last nationwide campaign, the two major parties raised and spent more money than ever before in support of their candidates and platforms. It seems clear that reports of the imminent demise of the two-party system are premature at best.

I would go with A and here's why:


Negate A
The amount of money raised and spent by a political party is NOT one valid criterion for judging the influence of the party

If u do this the argument falls apart (one of the premises stated that is used partly in the conclusion)since the argument says that
the two major parties raised and spent more money than ever before in support of their candidates and platforms and hence It seems clear that reports of the imminent demise of the two-party system are premature at best leading to the fact that the 2m ajor parties still influence the otcomes of elections.

So this choice must be true for the conclusion to stand.

B) A significant increase in the number of third-party candidates would be evidence of a decline in the importance of the two major parties.

ELIMINATE The argument talks about the winning of thrid parties nothing to do with a significant increase in the number of third-party candidates

(C) The two-party system has contributed significantly to the stability of the American political structure.

SO WHAT We cannot say its doing that today and in no way causes the conclusion to fall part.

(D) The mass media tend to favor an independent or third-party candidate over a candidate from one of the two major parties.

If u negate it it still doesnt cause the conclusion to fall apart and in fact it can strengthen the argument. We are talking about the strength of influence of 2 major parties

IRRELEVANT

(E) The mass media are relatively unimportant in deciding the outcome of most elections.

GREAT .One more strengthen the conclusion choice we will use it another day not for this one.



Regards,
CR

(C) The two-party system has contributed significantly to the stability of the American political structure.

SO WHAT We cannot say its doing that today and in no way causes the conclusion to fall part.

Perfect explanation by Cramya.

I would like to add something.

American Political Structure is actually out of scope. It was not mentioned that we were discussing about any particular country.
Correct me if I am wrong

Thanks Guys..

choice C should be the correct choice

I am pretty sure that the right answer is A.