Answer: Sunk cost
Explanation:
Sunk cost means the expense which has been already met by the firm and they cannot be recovered at any rate. Sunk costs are not based on the future decisions as these expenses for the firm are the same irrelevant to the project which it is assigned. Sunk costs are not a part of the budget plan.
In the given scenario the delivery company has spent $3500 in order to upgrade the truck. So $3500 is treated as sunk cost in the proposed project.