my take is B, here is why:

A. a greater proportion than it was: Negate this choice as it is singular and costs is plural
B. a greater proportion than: seems logical, lets keep this as an option
C. a greater proportion than they have been: need simple past here, negated
D. which is greater than was so: what is which referring to? and so is not idiomatic
E. which is greater than it has been: problem with which..it is singular..need simple past

by POE, B is the choice...OA please?

The answer is B.

A -- it's wrong, but not because costs is plural. 'it' in the phrase refers to the proportion of television costs. Therefore the problem is not agreement, but redundance since the pronoun unnecessarily refers back to a word in the sentence.

B -- it's correct because it eliminates the redundance issue in A.

C-- wrong verb tense

D -- 'which' is wrong

E-- problem with which and wrong verb tense

Thanks Danielle..looks like i am coming close

you totally are -- keep it up!