Which of the following forms is primarily used for reporting an employee's annual income to the IRS?

The form primarily used for reporting an employee's annual income to the IRS is Form W-2. This form is issued by employers to report wages paid to employees and the taxes withheld from those wages over the previous year. It serves as a key document for employees when filing their income tax returns, as it provides detailed information about their earnings and tax contributions.

Form I-9 is used for verifying the identity and employment eligibility of individuals hired for employment in the United States but does not report income. Form 1099 is utilized for reporting various types of income other than wages, such as payments made to independent contractors or interest income. Meanwhile, Form W-4 is the form that employees complete to indicate their tax withholding preferences to their employer, rather than reporting income directly. Therefore, Form W-2 is the correct answer for the purpose of reporting an employee's annual income to the IRS.