From N-400 instructions:
Required Evidence - Reduced Fee
1. A copy of each household member’s most recent Federal tax return;
2. If the household member did not file a Federal tax return, or if the tax return does not properly reflect his or her current income, submit copies of consecutive pay statements (stubs) for a minimum of the past month, a recent Form W-2, Form SSA-1099, or employer statements on business stationery showing salary or wages paid; or
3. If you are a student and not living with your mother or father or are not claimed as a dependent on your mother or father’s Federal tax return, do not include your mother’s or father’s incomes. You should only provide proof of your income or documentation that shows you are not required to file a Federal or state tax return, such as proof that you are a full-time student as supporting documentation.