What form must be included with an employee's federal, state, and city income tax returns?

The W-2 form is the correct choice because it is a crucial document that employers provide to employees at the end of the tax year. This form summarizes the total earnings an employee received during the year along with the amount of federal, state, and local taxes withheld. The information on the W-2 is necessary for employees when filing their income tax returns, as it informs the IRS and state tax agencies of how much income was earned and how much tax has already been paid through withholding.

In contrast, the other forms listed have different purposes: the W-4 is used by employees to indicate their tax withholding preferences to their employer rather than for filing taxes; the 1099 form is typically issued for independent contractors or freelancers to report income not subject to withholding; and Schedule C is used by sole proprietors to report business income and expenses. Therefore, the W-2 form is the only one that directly relates to an employee's income reporting for federal, state, and city tax returns.