Final answer:
The I-9 form is commonly known as the I-9 tax form and is used for verifying the identity and employment authorization of individuals hired for employment in the United States. It is not related to income tax.
Step-by-step explanation:
The tax form commonly known as the I-9 is the I-9 form itself. It is used for verifying the identity and employment authorization of individuals hired for employment in the United States. This form is not related to income tax, but rather it is used for immigration and employment eligibility purposes.
The other options listed in the question are all related to income tax forms. The W-2 form is used by employers to report wages paid to employees and the taxes withheld from their paychecks. The 1099 form is used to report income received outside of regular employment, such as freelance or contract work. The 1040 form is the standard individual income tax form used to report annual income and calculate the amount of taxes owed to the government.