Choice "C" is correct. Testing to see whether equipment listed in the accounting records is physically present in the plant and still in service is an effective way to test whether unrecorded disposals occurred.
Choice "a" is incorrect. Tracing from equipment found in service to accounting records would be performed to test the completeness assertion that all equipment on hand has been recorded, rather than testing the assertion that all recorded equipment is still in service.
Choice "b" is incorrect. Reviewing whether depreciation is still being taken would not provide evidence about unrecorded fixed asset disposals unless the auditor also performs a physical inspection of the assets being depreciated. Also, assets that are fully depreciated but still in service would not appear in depreciation records.
Choice "d" is incorrect. Examining additions to the other assets account would not be likely to lead to discovery of unrecorded fixed asset disposals, since newer assets are not likely to have been retired.