When forming an opinion on the adequacy of management's systems of internal control, which of the following findings would provide the most reliable assurance to the chief audit executive?

During an audit of the hiring process in a law firm, it was discovered that potential employees' credentials were not always confirmed sufficiently. This process remained unchanged at the following audit.

During an audit of the accounts payable department, auditors calculated that two percent of accounts were paid past due. This condition persisted at a follow up audit.

During an audit of the vehicle fleet of a rental agency, it was determined that at any given time, eight percent of the vehicles were not operational. During the next audit, this figure had increased.

During an audit of the cash handling process in a casino, internal audit discovered control deficiencies in the transfer process between the slot machines and the cash counting area. It was corrected immediately.