Choice "B" is correct. Completeness is the assertion supported by the auditor tracing the serial numbers on equipment (source) to a nonissuer's sub-ledger (book).Choice "c" is incorrect. Valuation and allocation relate to whether the amounts have been recorded correctly. Reviewing serial numbers on equipment would not provide information about the proper value of the equipment.Choice "d" is incorrect. Rights and obligations relate to whether the entity holds or controls the rights to the equipment. Inspecting supporting transactions or inspection of contracts would provide evidence of rights and obligations for equipment.Choice "a" is incorrect. Assertions about presentation and disclosure relate to making sure components of the financial statements are properly presented, described, and disclosed. The auditor would need to review the financial statements, not the serial numbers on equipment, to support the presentation and disclosure assertion.