My Professor changed my multiple choice answers. What should I do?
This is really tough to approach. On the one hand, it’s incredibly hard to prove this (also since there’s no clear incentive for the professor to do this). On the other hand, you stand to lose a lot if you’re wrong.

That said, paraphrasing Hanlon’s Razor: Never attribute to malice that which is adequately explained by incompetence. 

Your professor might have mixed up something, or accidentally erased your answers (and then wanted to err on your side). You can simply approach them and nicely ask about question 17, that you distinctly remember putting another answer, but you could be wrong etc.