Straight-line depreciation is a method of calculating the decrease in the value of an asset over its useful life. This method assumes that the asset will lose its value evenly over its useful life. To calculate straight-line depreciation, you divide the cost of the asset by its useful life. The result is a fixed percentage of the cost of the asset that is written off each year. Therefore, the correct option is: "a fixed percentage of the cost of the asset written off each year."