Final answer:
The W-2 form is considered an acceptable Unique Identification Number (UIN) as long as it has a full Social Security Number (SSN).
Step-by-step explanation:
The tax form that is considered an acceptable Unique Identification Number (UIN) as long as it has a full Social Security Number (SSN) is the W-2 form. The W-2 form is often used by employers to report an employee's annual wages and the amount of taxes withheld from their paycheck. It includes the employee's SSN, which can serve as the UIN for tax purposes.