Final answer:
Statement C is incorrect; existence assertions deal with the risk of overstatements in the financial statements by verifying the existence of assets and liabilities, and completeness assertions focus on preventing understatements by ensuring all transactions are recorded.
Step-by-step explanation:
The statement about the existence and completeness assertions that is not true is C) Existence deals with understatements and completeness deals with overstatements. In fact, existence deals with potential overstatements by verifying that assets or liabilities recorded in the financial statements actually exist at the balance sheet date, whereas completeness deals with the risk of understatements by ensuring that all transactions and accounts that should be recorded are indeed included in the financial statements. The completeness assertion specifically deals with the risk of unrecorded transactions which could lead to underreporting of liabilities and expenses or underreporting of assets and revenues.