I will also go for (B)..
During the 1980â€™s it became clear that for environmentalists who sought financial aid
, it was far more efficient to solicit private funds
than to go to state or federal agencies
Mark in Red are parallel infinitive Phrase.
Mark in Green maintains the comparison valid.
(D) and (E): Run-on Sentences.
(C): ....efficient to go to
than to go to......
Clause : "private organizations were far more efficient to go to
" ending with "to". I will not prefer clause or sentence ending with any prepostion.
(A): comparison is not clear: During the 1980â€™s it became clear that soliciting private funds was far more efficient for environmentalists who sought financial aid than to go to state or federal agencies
Mark is red is for whom? I guess we need some clause (with ellipsis or whatever)..
I hope I am right
.. Please feel free to correct me....