The correct answer is b) accumulated depreciation.
Accumulated depreciation is a contra asset account that is used to offset the value of fixed assets on the balance sheet. It represents the total amount of depreciation expense that has been recognized against an asset over time. By using this contra account, businesses can show the original cost of the asset while also reflecting its diminishing value due to wear and tear or obsolescence.
In contrast, options a) depreciation expense refers to the periodic expense recognized on the income statement, c) unearned revenue is a liability representing money received for services not yet performed, and d) book value is a term referring to the value of an asset according to its balance sheet account, which does not specifically indicate a contra account. Thus, accumulated depreciation is the only contra account among the options listed.