1. The problem asks about the classification of the book value of old equipment in a keep-or-replace decision. 2. In decision-making, costs and values are categorized as relevant or irrelevant based on whether they affect the decision. 3. The book value is the original cost minus accumulated depreciation; it is a historical cost. 4. Historical costs are considered sunk costs and do not affect future decisions. 5. Therefore, the book value is irrelevant to the keep-or-replace decision because it cannot be changed by the decision. 6. It is not incremental because it does not represent additional costs or benefits from the decision. 7. It is not an opportunity cost because it does not represent the value of the next best alternative. 8. Hence, the correct classification is "Irrelevant."