Which document is critical for tax preparation and indicates income and tax withheld?

The essential document for tax preparation that indicates income earned and taxes withheld is the Form W-2. This form is provided by employers to employees and details wages earned and the amount of federal, state, and other taxes withheld throughout the year. It is crucial for accurately filing income tax returns, as it ensures individuals report their income correctly and understand their tax obligations.

In comparison, while Form 1099 is used to report income other than wages, such as freelance or contract work, it does not include withholding information for taxes. Form 1040 is the actual tax return form that individuals complete to report their annual income and calculate their tax liability, but it relies on the information provided in W-2s and 1099s for income reporting. Form I-9 is used to verify an employee's eligibility to work in the United States and does not pertain to income or taxes. Thus, the W-2 is the primary document for employees to prepare their taxes effectively.