Answer: Choice C. Art not without ambition, but without / The illness should attend it.
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Explanation:
The first part "art not without ambition" basically means "you have ambition"; however, the second part contradicts it in the sense that Lady Macbeth is wondering out loud if he has the proper drive/motivation to get the job done, even if it means murder. To Lady Macbeth, she sees that the ends justify the means. In other words, the act of murder isn't so bad in her eyes as long as the goal of gaining the crown can be reached.
I'm not 100% sure here (my Shakespeare is a bit rusty) but I think "without the illness should attend it" is like saying "you don't have the mental (in)stability to be able to handle murdering someone". But I could be wrong on this interpretation.
