Final answer:
Accumulated depreciation is the contra account among the options given as it reduces the value of a related asset account on the balance sheet.
Step-by-step explanation:
Among the options provided, Accumulated Depreciation is a contra account. A contra account is an account found on the balance sheet that reduces the amount of a related account. Accumulated depreciation is used to reduce the value of the property, plant, and equipment on a company's balance sheet, representing the amount of an asset's value that has been used up over time.
In contrast, Unearned Revenue is a liability account that reflects funds received for services or goods not yet delivered; Patents are intangible assets, and Premium on Bonds Payable is an account that reflects the additional amount that investors are willing to pay above the face value of the bonds.