Choice "D" is correct. The bonds' book value equals the bond face value less the unamortized discount. The unamortized discount$10,000 - $2,000$8,000, so the book value equals face value - $8,000. The selling price of the bonds is face value + $14,000. The gain on the sale equals the selling price (face value + $14,000) minus the book value (face value - $8,000)$22,000.
Choice "c" is incorrect. The premium less the amortized discount is not the proper calculation of the gain.
Choice "a" is incorrect. This calculation ignores the $2,000 amortization of discount.
Choice "b" is incorrect. In this calculation, the $2,000 amortization of discount is included as a decrease in book value rather than an increase.