The correct answers are: Future costs, incremental costs and differential costs.
A decision is about the future, therefore relevant costs are future costs.
If a cost is unavoidable then any decision taken about the future will not affect the cost, therefore unavoidable costs are NOT relevant costs.
Incremental costs are extra costs which will be incurred in the future therefore relevant costs are incremental costs.
Differential costs are the difference in total costs between alternatives and they are therefore affected by a decision taken now and they are associated with relevant costs.
