@Chaitanya_1986 - Choice A is not correct. The correct answer is B.

Soaring television costs accounted for more than half the spending in the presidential campaign of 1992, a greater proportion than it was in any previous election.

A. a greater proportion than it was
This sentence has pronoun reference error. The pronoun "it" can only refer to "spending in the presidential campaign" or to "presidential campaign". Both these references are non-sensical in the context of this sentence.

B. a greater proportion than
This option compares spending over two election periods. Correct.

C. a greater proportion than they have been
The only plural referent for "they' is "soaring television costs". This will result in is an illogical comparison.

D. which is greater than was so
The reference made by "which modifier" is not clear. Is "which" referring to "presidential campaign of 1992" or spending in presidential campaign of 1992" or so on. Thus, use of which modifier is ambiguous.

E. which is greater than it has been
Same as D.