Answer:
a. book value of an asset less its salvage value.
Step-by-step explanation:
The depreciable cost is the cost in which the salvage value or the residual value is deducted from the book value of an asset
In mathematically,
Depreciable cost = Book value of an asset - salvage value
And, the same is used for computing the depreciation expense using the straight line method or units of production method