Which of the following best describes 'fixed costs'?

Fixed costs are expenses that do not change with the level of production or output. This means that regardless of how much a company produces, these costs remain unchanged over a certain range of activity. Examples of fixed costs include rent, salaries of permanent staff, insurance, and depreciation on equipment. Since these costs do not fluctuate with production levels, they play a crucial role in understanding a company's financial stability and planning.

The other descriptions do not accurately reflect fixed costs. Costs that vary directly with production levels are known as variable costs, while costs that fluctuate with market conditions would be more related to market-driven expenses. Lastly, the option that includes both variable and fixed components refers to a mixed cost, which combines characteristics of both fixed and variable costs. Therefore, the description of fixed costs as something that remains constant regardless of output levels is the most accurate.