I hope you will help me with this important question.

In short, my niece was 15 years old when her mother was naturalized to become US citizen. She is now 19 years old. As we speak, she doesn't have any proof of the citizenship through her mother..

We were thinking to file for N-600 but looks like that process could take more than three months..

However, filing for US passport might be short route because processing time to obtain the passport takes less than three weeks..

If we decide to do passport application, what document is she supposed to produce as supporting evidence to get her passport processed? Please advise

Thank you so much for your help.