Choice "B" is correct. Estimated restoration costs should be added to the depletable base of the natural resource. In this way, the amount of depletion charged to expense over the life of the mining operation will include the restoration costs.
Choice "c" is incorrect. Restoration costs should not be expensed as incurred.
Choice "d" is incorrect. While the costs are capitalized (by being added to the depletion base of the mine), they are depleted, not depreciated.
Choice "a" is incorrect. The costs should be added to the depletion base of the mine, not subtracted, as they would not have been initially included in the depletion base of the mine.