Which of the following payments is not taxable for FICA?

Wage supplements designed to cover the difference between employees' salaries and military pay are not taxable for FICA (Federal Insurance Contributions Act) purposes because they are typically categorized as benefits rather than wages for employment. When employees are called to active military service, employers may offer these supplements to ensure their income remains stable. Since these payments are not technically considered compensation for services rendered but rather support for employees during a specific circumstance, they are exempt from FICA taxation.

In contrast, back-pay awards, severance pay, and retroactive wage increases are all considered forms of compensation for services, which makes them taxable under FICA. These types of payments directly relate to employment and are intended as remuneration for work completed or as a result of employment termination, hence they fall under FICA's purview. Understanding the distinction between what constitutes wages versus benefits is crucial in determining taxability under FICA.