Choice "C" is correct. Variable costs change with the level of output but may not change purely in response to different selected alternatives. Although variable costs are frequently relevant, they are not always relevant. Relevant costs are those costs that will change in response to the selection of different courses of action.
Choice "b" is incorrect. Incremental costs represent the change in cost associated with different alternatives and are considered synonymous with relevant.
Choice "a" is incorrect. Differential costs represent the change in costs associated with two separate courses of action and are considered synonymous with relevant.
Choice "d" is incorrect. Avoidable costs represent the costs that can be averted by selecting different courses of action and are considered synonymous with relevant.