Final answer:
A change in the salvage value of machinery due to the price increase of scrap metal is a Change in an accounting estimate, as it updates the expected value of the asset at the end of its useful life based on new market information.
Step-by-step explanation:
The change in the salvage value of machinery due to an increase in the price of scrap metal is categorized as B. Change in an accounting estimate. Accounting estimates are adjustments that companies make to certain figures within their financial statements, where exact amounts cannot be known and must be estimated based on judgment and available information. The increase in the price of scrap metal would directly affect the salvage value of machinery, thus necessitating an update to the previously estimated salvage value. This change is a normal part of business operations and reflects new information influencing the valuation of the company's assets.