C is correct.
Praet, how does "only" change the meaning in D ? Somehow I don't see it...
Efforts only have been...partially successful ..Bad
This quote suggest that ONLY EFFORTS have been partially successful.
This suggests...Several things have been attempted to reduce the problem, but out of those, "efforts only"
have been partially successful.C
Efforts have been only partially successful. Good
Here the sentence correctly conveys the idea that efforts to reduce it have been ONLY partially successful