I agree with you, Molly. Saying "do a mistake" is not a mistake. It is a failure to use the most common idiom. When people talk about linguistic errors they often fail to specify what sort of error they mean. Sometimes people use "error" to describe things that just don't sound good to them. That, in my opinion, is a mistake.

Hi Molly, B is incorrect. "Mistake" takes the verb "make", not "do", so it should read as "...I haven't made many mistakes..."