I agree with David's assessment in general. I agree that 'Wouldn't you?' would not be natural for the intended purpose.
It would be completely natural to word it this way:
A: I wouldn't do it if I were you. B: You wouldn't? Why (not)?
The words 'You wouldn't' are not really a question in this case -- though the intonation would be that of a question. This is more an utterance of surprise using a partial repetition of what speaker A has just said. In this case, I don't see the use of 'not' after 'Why' as absolutely necessary since the idea of 'not' is already included in 'You wouldn't. Having said that, I think most people would say 'Why not'?
The above issue is what I read from a test. The 4 options are as exactly as are stated above. I had to think a lot, considered each option, eliminated one after one,and finally decided the best option in this case was "Wouldn't you? Why?", which is the official answer.
I just wanted to share and wish those test makers could read this and would be more careful later. Once again, thank all of you so much for the contributions.
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