Choice "C" is correct. Opportunity cost is the next best use of productive capacity. The production that is forfeited to produce the special order is referred to as the next best alternative use of the facility.Choice "a" is incorrect. The net present value is the difference between the cost of an investment and the present value of its cash flows, not the opportunity cost.Choice "b" is incorrect. The internal rate of return is the computed rate at which net present value is zero. It is not the opportunity cost.Choice "d" is incorrect. Depreciation expense is reasonable and rational allocation of cost over time. It is not opportunity cost.