The correct answer is D. wages.
A gratuity is a gift or payment made voluntarily or without obligation. It is often given in recognition of good service or as a thank-you. Wages, on the other hand, are payments made to employees in exchange for their work. They are usually paid on a regular basis, such as weekly or monthly.
An annuity is a series of regular payments made at fixed intervals, typically for the rest of the recipient’s life. A stipend is a fixed sum of money paid regularly to someone, especially as a grant or allowance. A discount is a reduction in the price of something.
In conclusion, the antonym of gratuity is wages. This is because a gratuity is a gift or payment made voluntarily or without obligation, while wages are payments made to employees in exchange for their work.