Answer: A "Ouch!"
Step-by-step explanation: Option B is discarded because the quote must end before the narration ("Lou cried") resumes; option C is wrong because you can't mix single and double quotation marks at the same level; option D could be correct only in a context where the exclamation point is part of the main sentence and not of the quote, e.g.: I can't believe I slapped Walter in the face and he didn't even say "Ouch"!