Explanation
Requiring written authorization for all payment transactions is the IS auditor's best recommendation for a compensating control in an environment where segregation of duties (SoD) cannot be enforced in an accounts payable system. SoD is a principle that requires different individuals or functions to perform different tasks or roles in a business process, such as initiating, approving, recording and reconciling transactions. SoD reduces the risk of errors, fraud and misuse of resources by preventing any single person or function from having excessive or conflicting authority or responsibility. A compensating control is a control that mitigates or reduces the risk associated with the absence or weakness of another control. Requiring written authorization for all payment transactions is a compensating control that provides an independent verification and approval of each transaction before it is processed by the accounts payable system. This control can help to detect and prevent unauthorized, duplicate or erroneous payments, and to ensure compliance with policies and procedures. The other options are not as effective as option A, as they do not provide an independent verification or approval of payment transactions. Restricting payment authorization to senior staff members is a control that limits the number of people who can authorize payments, but it does not prevent them from initiating or processing payments themselves, which could violate SoD. Reconciling payment transactions with invoices is a control that verifies that the payments match the invoices, but it does not prevent unauthorized, duplicate or erroneous payments from being processed by the accounts payable system.
Reviewing payment transaction history is a control that monitors and analyzes the payment transactions after they have been processed by the accounts payable system, but it does not prevent unauthorized, duplicate or erroneous payments from occurring in the first place. References: CISA Review Manual (Digital Version) , Chapter 5: Protection of Information Assets, Section 5.2: Logical Access.