Sodium acetate/acetic acid is a buffer solution. The pH of 0.1M acetic acid is about 2.9. If you have 0.1M acetic acid with 0.1M sodium acetate, the pH is about 4.7. I expect the answer is C. Another way to look at it is that you are adding a base to an acid so the pH should become more basic; i.e., the pH will increase.