Final answer:
The furnace is best described as a sunk cost because it has already been purchased and cannot be recovered.
Step-by-step explanation:
The furnace is best described as a sunk cost. A sunk cost is a cost that has already been incurred and cannot be recovered. In this case, Weston Steel purchased the furnace six years ago for $2.2 million, but now it cannot be used or sold due to changes in emission requirements.