Choice "D" is correct. The explanation that plant assets were retired during the year would most likely satisfy an auditor who questions management about significant debits to accumulated depreciation accounts made during the year. The journal entry to retire an asset includes a debit to accumulated depreciation and a credit to the asset account.Choice "c" is incorrect. If prior years' depreciation expense was erroneously understated, the auditor would expect a debit to retained earnings and a credit to accumulated depreciation for the error correction.Choice "b" is incorrect. Typically, accumulated depreciation is debited when there is total and permanent impairment, and when an asset is sold, retired, or otherwise disposed of. If the current year's depreciation expense was erroneously understated, this would not typically result in debits to accumulated depreciation. Instead, the auditor would expect smaller amounts credited to accumulated depreciation.Choice "a" is incorrect. A change in useful life is handled prospectively. An increase in the remaining useful lives of assets would result in smaller amounts of depreciation expense being credited to accumulated depreciation.