My answer is: B.
I eliminated choices 'D' and 'E', because which is... seems to incorrectly refer to the presidential election itself, instead of the costs associated with the election. Even if we tie this phrase back to the costs, the word "costs" is plural, while the word "is" refers to the singular case (if anything, "are" should be used).
I eliminated choice 'A' because of the subject-verb (dis)agreement between "costs" (plural) and the "it" (singular) in "than it was".
I eliminated choice 'C' because, while "they" does agree with "costs", the usage of "have been" suggests that the costs continue to soar. However, the sentence is comparing the costs of the 1992 election with elections prior to 1992. Either way, the events have concluded. I'd have more difficulty eliminating 'C' if the phrase was "...they HAD been".
For all I know my reasoning above is faulty, but this is how I analyzed the question.