Which is an example of mixed costs?

Mixed costs, also known as semi-variable costs, consist of both fixed and variable components. The provided choice of a utility bill with a base charge and usage fee perfectly exemplifies this concept.

In this case, the utility bill includes a base charge that remains constant regardless of usage, representing the fixed portion of the cost. In contrast, the usage fee varies with the amount of utilities consumed, representing the variable component. This combination of a fixed base charge and a variable usage fee illustrates how mixed costs operate, making option B the correct answer.

Other options don't combine fixed and variable elements in the same manner. A flat salary represents a purely fixed cost since it does not change with production or activity levels. A one-time fee for a service is typically a single, fixed cost without variability. The idea of "only the variable part of a cost" does not encompass the fixed aspect required to classify it as a mixed cost. Therefore, the correct identification of mixed costs is aptly demonstrated by the utility bill option.