(C) "they" would have to refer to "soaring television costs", by elimination: there aren't any other plural nouns.

literally, this makes no sense, since television costs weren't "soaring" in OTHER elections.
(note that you MUST take the pronoun to stand for "soaring television costs"; you are NOT allowed to extract just "television costs" and pretend that the pronoun stands only for that.)

"have been" is an even bigger problem, though, since it implies the presence of "accounting". you can't do this unless the word "accounting" is actually present elsewhere in the sentence; it isn't.

actually, for those choices to make sense, "it" would have to be "proportion". (the proportion was greater in '92 than it was in any previous election.)

if "it/they" = "the cost(s)", then it's nonsense to use "was/were". you'd have to have "represented", "accounted for", "amounted to", etc.
'they have' or 'they did' can't make this option acceptable because "have"/"did" can't stand for "accounted for".