Which deduction from gross pay is considered a non-mandatory expense?

The correct choice of trade union fees as a non-mandatory expense stems from the nature of such deductions. Trade union fees are paid voluntarily by employees who choose to become members of a trade union, which advocates for workers' rights and interests. Unlike other deductions, trade union fees are not legally required; rather, they are dependent on an individual’s decision to join and support the union.

In contrast, income tax and national insurance contributions are mandatory deductions that are required by law for all employees who meet the specified income thresholds. These taxes fund public services and social security benefits, making them obligatory contributions. Similarly, student loan repayments become mandatory once certain income levels are reached, as they are tied to a specific government-mandated repayment plan.

Therefore, trade union fees stand out as a choice that reflects discretionary spending, aligning with personal employment decisions and membership in a specific organization, making it the appropriate answer among the given options.