Explanation

 "She is a nice girl." is a positive statement.

The auxiliary verb here is "is" (from "is a nice girl").

To form the tag question, we invert the auxiliary verb and add the corresponding pronoun.

Thus, the tag question is:

"She is a nice girl, isn't she?"

This structure follows the rule: positive statement + negative tag.

Therefore, the correct answer is:  Isn't she