Which of the following is NOT a type of withholding rate mentioned?

The correct answer, which identifies the option that is NOT a type of withholding rate, is based on understanding the different categories of withholdings that relate to employee compensation and taxation.

Supplemental withholding rates, which include both under and over $1 million, refer specifically to additional income that is taxed at a rate set by the IRS, which can vary depending on the overall compensation scenario. The supplemental withholding rate is often applied to bonuses and other forms of supplemental income. On the other hand, the Social Security tax rate is a mandatory payroll tax rate that applies to wages up to a certain limit and is part of the Federal Insurance Contributions Act (FICA). It is a well-defined withholding that affects both employers and employees.

Estate tax withholding, however, pertains to taxes collected from the estates of deceased individuals rather than a withholding from employee wages or bonuses during their lifetime. This is a distinct type of taxation and does not fit within the common frameworks of payroll withholding rates used for current employee compensation. Thus, it stands out as the answer that does not align with the others, which all pertain to standard payroll practices.