The conclusion of this argument is the first sentence: "The increase in the minimum wage in Country X will quickly lead to a decrease in Country X’s rate of unemployment." The subsequent sentences explain why the increase in the minimum wage will lead to a decrease in the rate of unemployment.
Notice that this is a Weaken EXCEPT question. That means that the four incorrect answers will weaken the argument and we are looking for the one correct answer which does NOT weaken. So answer choice (D) does
weaken the argument, which is why it is incorrect--we're looking for an answer choice that does NOT weaken it. Answer choices (A), (B), (D), and (E) all weaken the argument. Answer choice (C) is the one which does not weaken the argument.
As for why answer choice (C) does not weaken the argument, check out Paul's post above:
(C) only tells us that some factory workers are paid more than minimum wage. Like Adam mentioned above, this ignores both other factory workers (who do make minimum wage) as well as all non-factory workers. But just as importantly, the premises of the stimulus already tell us that "Raising the minimum wage will lead to more disposable income for a large segment of the working population". (As always, for Weaken questions we're not worried about attacking the premises, rather we're finding an issue with how the conclusion is drawn out of those premises). So since we've already established that raising the minimum wage will mean more $ for a large segment of workers, who cares if some factory workers aren't affected? That doesn't weaken the argument.
Hope this helps!