Final answer:
The formula for straight-line depreciation is (cost + residual value)/service life.
Step-by-step explanation:
The formula for straight-line depreciation is OPTION C: (cost + residual value)/service life. In straight-line depreciation, the cost of the asset is spread evenly over its service life. The formula reflects this by adding the initial cost of the asset to the residual value (the estimated value at the end of its useful life) and then dividing it by the total service life in years.