The correct answer is a) mixed cost.
Mixed costs, also known as semi-variable costs, are expenses that contain both fixed and variable components. This means that part of the cost remains constant regardless of production or activity level, while another part varies with the level of production or activity. For example, a utility bill may have a base charge (fixed component) and a charge that increases with usage (variable component). Understanding mixed costs is crucial for budgeting and financial analysis, as it allows businesses to forecast expenses more accurately based on expected activity levels.