The auditor should be most concerned about the information security policy not being approved by the policy owner. This is because the policy owner is the person who has the authority and accountability for ensuring that the policy is implemented and enforced. Without the policy owner's approval, the policy may not reflect the organization's objectives, risks, and compliance requirements. The policy owner is usually a senior executive or a board member who has a stake in the information security governance. The other options are less critical than the policy owner's approval, although they may also indicate some weaknesses in the policy development and maintenance process. References:
* CISA Review Manual (Digital Version), Chapter 1, Section 1.21
* CISA Online Review Course, Domain 5, Module 1, Lesson 12

The best way to assess the effectiveness of changes made to processes and tools related to an organization's BCP is to review the full test results of the BCP. Full test results can provide evidence of whether the changes have improved the BCP's objectives, such as recovery time objectives (RTOs), recovery point objectives (RPOs), and business impact analysis (BIA). The other options are not as effective as reviewing the full test results, as they do not demonstrate the actual performance of the BCP under simulated disaster scenarios.
Completed test plans are only documents that outline the scope, objectives, and procedures of the BCP testing, but they do not show the outcomes or issues encountered during the testing. Updated inventory of systems is a component of the BCP that identifies the critical systems and resources required for business continuity, but it does not measure the effectiveness of the BCP changes. Change management processes are controls that ensure that changes to the BCP are authorized, documented, and communicated, but they do not evaluate the impact or benefit of the changes. References: CISA Review Manual (Digital Version), Chapter 4, Section 4.2.3

A computer-assisted technique is the most helpful method for an IS auditor to determine whether duplicate vendor payments exist on a complex system with a high volume of transactions. A computer-assisted technique is a tool or procedure that can be used to perform audit tests or procedures on data stored in electronic form. Examples of computer-assisted techniques include data analysis software, query tools, scripting languages, and specialized audit software. A computer-assisted technique can help an IS auditor to identify and extract duplicate payments from a large data set, perform calculations and comparisons, and generate reports and summaries. A computer-assisted technique can also provide more accuracy, efficiency, and coverage than manual methods.
Stratified sampling, statistical sampling, and process walk-through are not as helpful as a computer-assisted technique for this purpose. Stratified sampling is a sampling method that divides the population into subgroups based on certain characteristics and selects samples from each subgroup. Statistical sampling is a sampling method that uses probability theory to determine the sample size and selection criteria. Process walk-through is a review technique that involves following a transaction or process from start to finish and observing the inputs, outputs, controls, and documentation. These methods may be useful for other audit objectives, but they are not as effective as a computer-assisted technique for detecting duplicate payments in a complex and high-volume system. References: ISACA Frameworks: Blueprints for Success, [ISACA Glossary of Terms]

The auditor's best recommendation prior to go-live is to conduct a mock conversion test. This is because a mock conversion test can help to verify the accuracy, completeness, and validity of the data conversion process. A mock conversion test can also help to identify and resolve any issues or errors before the actual conversion takes place. A mock conversion test can also provide assurance that the converted data meets the business requirements and expectations. References:
* CISA Review Manual (Digital Version), Chapter 3, Section 3.3.21
* CISA Online Review Course, Domain 2, Module 2, Lesson 22