I think she looks pretty with all of them, but I especially like the pixie cut. She looks so elfin, which is a very beautiful look to me. The haircut also shows off her face best. I actually think she would look best with the pixie hair cut in the same color as the third picture. Blonde seems to suit her much better than that dark brown. The pixie cut + blonde combo would be perfect.
This question really shows you how important a haircut really is. I voted for #2, where she looks GORGEOUS. #3 she looks good but the long darker hair with the part really suits her face. Conversely in the 2 short hairstyles she looks an astonishingly large amount worse, the difference between the first 2 choices is immense. She looks like a 9/10 in the #2 pic and like a 3/10 in the first pic.
TL;DR version: Short hair looks bad on women, don't do that. #2 is best for her