My answer was E, but the correct answer was C. I tried to figure out why C is right and E isn't, but to me the difference between them seems pretty subtle. I'll share my analysis here, and it would be great if you can comment on it while going over the question in general. Thanks in advance!
So imo:
C) is the right answer, because Darla doesn't really dispute Charles's conclusion itself. Instead, Darla gives another view, based on the same given stiuation - which actually weakens Charles's argument.
D) isn't quite right, since Darla doesn't really attack Charles's conclusion itself. And Darla doesn't clearly present her own argument. Darla just challenges Charles's conclusion and assumption by giving a different side of the issue.