Here's the
official explanation provided by the GMAC for this question:
The plural subject of this sentence,
efforts,. does not agree with the singular
verb has… reduced. Efforts requires the plural verb
have reduced..
Option A: Has reduced does not agree with
efforts Option B: Subject and verb do not agree; changes make the construction wordy
Option C: efforts does not agree with
has reduced; made a significant reduction in is wordy
Option D: In this clear, concise sentence,
efforts agrees with the plural verb
have… reduced.
Option E: E This alternative is wordy and awkward
The correct answer is D.
Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.