Complete the sentence with the most appropriate word or phrase out of the given options.
She did not mention that to Peter, ____?

did she

The most appropriate word or phrase to complete the sentence is did she.

The other options are not as accurate. Should she suggests that the speaker is asking whether or not she should have mentioned it to Peter. Did not she is a double negative, and it is not grammatically correct. She did is a statement, and it does not make sense in the context of the sentence. Did she is a question, and it is the most natural way to complete the sentence.