Choice "B" is correct. In this question, the total cost of the mine is the purchase price ($2,000,000) plus the preparation ($500,000), net of the anticipated sales price at the end of its useful life ($100,000). The total cost is thus $2,400,000 ($2,000,000 + $500,000 - $100,000$2,400,000). That total cost is divided by the estimated 750,000 tons of coal, for a per unit depletion amount of $3.20. Note that the depletion amount per unit would be the same for every year unless additional expenditures were incurred. Note that the 15,000 tons produced and sold is irrelevant because the question is asking for the depletion per ton. Often, the questions indicate that a certain number of units is produced and that a different amount is sold and ask about the depreciation expense for the period. In those questions, it is the number of units sold that count.
Choice "d" is incorrect, per the above calculation.
Choice "c" is incorrect, per the above calculation.
Choice "a" is incorrect. This answer ignores the anticipated sales price at the end of its useful life. This amount is effectively a salvage value and should be subtracted in the numerator of the depletion formula.