Niner710,
Here is how i went about it.

A. has significantly reduced advertising income both for business journals as well as general consumer magazines, especially if focusing (both..and.. is the correct idiom).

B.has significantly reduced advertising income both for business journals and for general consumer magazines, especially those focusing ( here ' Those' refers to both business journals and consumer magazines. ie. it refers to the word 'Both'.
consumer magazines and Business journals talk about technology, the author here is implying that he is referring to 2 items included rather than any specific one. That is the main reason he keeps both the items in plural state in order to avoid confusion over the word 'those')

C.significantly reduced advertising income for both business journals and for general consumer magazines, especially when focused (parallelism not maintained. Both X and Y is the correct idiom)

D.reduced both business journals’ and general consumer magazines’ advertising income significantly, especially if focused (wrong use of conditionals)

E.reduced advertising income significantly for both business journals, as well as for general consumer magazines, especially those focusing (wrong use of idiom. it should be Both X and Y)

So you see even if you could not identify 'Those' referring to 2 items, you still can arrive at the answer by negating other answer choices.

Hope this helps!