Hey guys,
I think you are both asking the same question, i.e. what makes answer choice (D) incorrect. I hope you both understood, from my explanation above, why answer choice (C) is correct - it correctly captures the Error in the Use of Evidence, i.e. the erroneous belief that weakening an argument in support of a given position actually disproves this argument.
Answer choice (D) outlines a possible assumption ("presumes..."), namely, that the argument cited is the union leaders'
only argument for their view. Does the author need to assume that the union leaders haven't made any other arguments defending their view? Of course not. Even if they have made other arguments, that wouldn't weaken the politician's conclusion. After all, the union leaders are biased, says the politician, so whatever arguments they make can theoretically be dismissed for the same reason as the argument cited in the stimulus. Since the logical opposite of the alleged assumption in answer choice (D) does not weaken the politician's argument, answer choice (D) does not contain an assumption upon which his argument depends.
ellyb - your approach to answer choice (D) is incorrect. This is not a Weaken question - we can't take it for granted that the politician actually made this assumption. Rather, for answer choice (D) to be correct, you need to
prove that this assumption was, in fact, made. It was not.
Hope this helps!
Thanks,