Variable pay systems are best described as:

Variable pay systems are designed to link an employee's compensation directly to their performance or the performance of the organization as a whole. This approach encourages employees to strive for higher levels of productivity and success, as their earnings can increase based on their contributions.

In this context, a type of commission based on performance typically exemplifies variable pay since it directly rewards employees for achieving certain goals or sales targets. As employees perform better, their commissions grow, creating a strong incentive for continued high performance.

The other options describe fixed or mandatory forms of compensation that do not incorporate performance metrics. For example, a fixed salary based on the position does not change and is not tied to individual output or results. Benefits that do not change with performance also represent fixed compensation rather than a link to performance outcomes. Lastly, a mandatory payment for all employees would suggest a standard salary or wage not influenced by individual performance, making it distinct from the variable pay structure.