Soaring advertising costs accounted for more than a third of expenses

The right answer here is D. The main concepts being tested here are 'Subject Verb Agreement' (really singular/plural) and 'Tenses'.

The first thing to note is that the "more significant portion" refers to the advertising costs, which is a plural subject. Immediately, this helps us eliminate A, B, E.

In C, the use of "they have been" doesn't make sense, as the costs of years gone by don't continue to be the costs that impact the current situation, they are simply something that happened in the past. Had this option said "than they were" it would have been fine, but it doesn't, and is therefore OUT

This leaves D as the sole remaining right answer. One thing to note is that in D, the subject if not referred to again. However, since there was no other viable noun/noun phrase, it is still clear what is being referred to in this sentence.

- Matoo