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The 4,500 pounds of Kaylene purchased in the past is not relevant to the decision. Relevant information for decision-making is expected future revenues and costs. Furthermore, the problem does not indicate how much of the 4,500 pounds purchased is still on hand, so it is not possible to know how much would remain after using 4,200 pounds for the special order.
Relevant information for decision-making is expected future revenues and costs. The relevant cost is the current price, because that is what it would cost to replace any Kaylene used from inventory and also the cost to purchase Kaylene today if needed for current production. The price paid 2 months ago has no bearing on the decision to be made.
Relevant information for decision-making is expected future revenues and costs. Historical prices are not relevant to the decision to be made.
The potentially remaining 300 pounds of Kaylene is not relevant. The problem does not indicate how much of the 4,500 pounds purchased is still on hand, so it is not possible to know how much would remain after using 4,200 pounds. The current market price is absolutely relevant, because that is what it would cost to replace any Kaylene used from inventory and also the cost to replace any Kaylene used from inventory and also the cost to purchase Kaylene today if needed for current production. The variable cost of the special order would need to include the cost of the material at its current price.