What documents must be completed by an employee to ensure correct tax withholding?

To ensure correct tax withholding for an employee, the appropriate document is IRS Form W-4. This form is specifically designed to help employers determine the right amount of federal income tax to withhold from an employee's paycheck based on their tax situation.

When an employee completes the W-4, they can provide information such as their filing status, number of dependents, and any additional amount they want withheld. This information is crucial for the employer to calculate the correct withholding amount accurately, making it essential for tax compliance and the individual's personal financial planning.

In contrast, IRS Form I-9 is not related to tax withholding; instead, it is used to verify an employee's eligibility to work in the United States. IRS Form W-5 is a request for advance payment of the earned income credit, which is different from withholding allowances. An Employee Contract Agreement is also not related to tax withholding; it is a document that outlines the terms of employment between the employer and the employee. Thus, opting for the W-4 is the correct approach to ensure proper tax withholding.