Choice 'a' is correct. Over the first 4 years, the asset would be depreciated down to $2,400. Once it was determined that the asset would last for another 4 years, $600 would be depreciated each year of that 4 year period. This change is a change in accounting estimate (the estimate being the life of the asset).

Changes is accounting estimate are accounted for in the current year and future years if the change affects both.

Choice 'b' is incorrect. This answer is the annual difference between the depreciation expense IF depreciation expense had been retroactively restated ($24,000 / 8 = $1,500) and the correct depreciation expense. Retroactive restatement is not appropriate for changes in accounting estimate.

Choice 'c' is incorrect. This answer is the depreciation expense IF depreciation had been retroactively restated ($24,000 / 8 = $1,500). Retroactive restatement is not appropriate for changes in accounting estimate.

Choice 'd' is incorrect. This answer is the undepreciated amount at the beginning of the fifth year or the amount of the annual depreciation expense for each of the first 4 years. Either way, it certainly is not going to be the depreciation expense for that year because the remaining cost will depreciated over the remaining period.