I think this is a poorly written question -- I too, thought the answer was A because it makes the premise of the argument incorrect. Like others before have stated, if the argument stated that wind power was the best choice for rural areas because it was cheaper, than C would be the best answer.
Upon further research, I found the original question, which includes an explanation for the answers A and C:
This is a weaken question. The shift from the first sentence to the second makes the assumption clear. The argument relies on the underlying claim that, because wind power produces the fewest pollutants, it is the best choice. More probably, there are other considerations. The correct choice will likely point out some other consideration that may outweigh the level of pollutants.
Sure, there are other considerations, but we don't know what other considerations would make something better or worse; therefore, we cannot use other considerations to weaken our argument. For all we know, a higher cost could make something better (in terms of quality, for example -- but we would be making an assumption here, too).
(A) A reference to an earlier (or different) report is almost always wrong; we have no way of comparing the validity of the two reports. Anyway, it doesn't address the assumption.
It's a huge assumption to think that an earlier report would almost always be wrong just because it came before the argument's report. And it's not true that we don't know the validity of the two reports; we must conclude that the report in Answer A is just as valid, if not more, than the report from the argument because the question says "if true".
(C) This is correct. This is a strong reason why wind power may not be the best choice for new rural areas.
And again, for C, wouldn't we be making an assumption to say that wind power is not the best choice just because it is more expensive? There are plenty of things that are more expensive, but "better". Usually, when we say something is better, we have certain criteria -- in this case, our criteria for "better" is the amount of pollutants it produces. Based solely on this information, and if we know that solar power produces less pollutants than wind power, then we can conclude that solar power is actually the best option.
Of course, maybe I just have a bias against the question now because I got it wrong
EDIT: after thinking about it, making the premise not true is not the most effective way to weaken an argument. Attacking assumptions in the argument is the better strategy, but again, this question is badly worded. It perhaps might be better written as such:
An Environmental Impact Coalition report shows that wind power produces the fewest pollutants among all
energy alternatives. Since rural areas cannot afford most conventional energy alternatives
, EnergyMill wind turbines are the best choice for powering new developments in rural areas.
Which of the following, if true, most seriously weakens the argument in the advertisement?
(A) An earlier Environmental Impact Coalition report showed that solar power produces fewer pollutants than does wind power.
(B) In the last five years, the government has invested more in wind power than in any other form of green energy.
(C) The dispersion of homes in rural areas makes wind power several times more expensive that conventional energy sources.
(D) The difference between the number of pollutants generated by wind power and conventional power sources is quite pronounced.
(E) The Environmental Impact Coalition issues reports only once a year.