Concept:The straight-line method of depreciation spreads the cost of an asset evenly over its useful life.Explanation:The formula for annual depreciation using this method is:Estimated useful lifeCost of asset−Scrap valueHere, the cost of the asset is reduced by the scrap value.The result is then divided by the estimated useful life of the asset.Therefore, the denominator in the formula represents the estimated useful life of the asset.Answer:C. estimated useful life of the asset.