I would say that (d) would be the best response. You usually reply 'Don't mention it' when someone thanks you. I have never personally heard this response as a reply to an apology. However, if someone said this as a reply to an apology, it would not sound strange. That is, I would understand this reply as saying 'I accept your apology'.
However, as this is probably more of an American idiom (I am Australian), I could not say for sure.
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