The best way to mitigate risk to an organization's network associated with devices permitted under a BYOD policy is to implement a network access control system, as this will allow the organization to monitor, authenticate, and authorize the devices that connect to the network, and to enforce security policies and compliance requirements12. A network access control system can help to prevent unauthorized or compromised devices from accessing sensitive data or resources, and to detect and isolate any potential threats or vulnerabilities34.
References
1: Network Access Control (NAC) - ISACA 2: Network Access Control (NAC) - Cisco 3: BYOD Security Risks: 6 Ways to Protect Your Organization - ReliaQuest5 4: How to Mitigate BYOD Risks and Challenges - CIOReview6

Explanation
The most important thing for an IS auditor to verify when evaluating an organization's data conversion and infrastructure migration plan is that a rollback plan is included. A rollback plan is a contingency plan that describes the steps and actions to be taken in case the data conversion or infrastructure migration fails or causes unacceptable problems or risks. A rollback plan can help to restore the original data and infrastructure, minimize the impact on the business operations and functions, and ensure the continuity and availability of the IT services. The IS auditor should verify that the rollback plan is feasible, tested, documented, and approved, and that it covers all the possible scenarios and outcomes of the data conversion or infrastructure migration. The other options are not as important as verifying the rollback plan, because they either do not address the potential failure or disruption of the data conversion or infrastructure migration, or they are part of the normal planning and execution process rather than a contingency plan. References: CISA Review Manual (Digital Version)1, Chapter 4, Section 4.2.3

Section: Protection of Information Assets
Explanation:
Depending on the complexity of an organization, there could be more than one plan to address various
aspects of business continuity and disaster recovery. These do not necessarily have to be integrated into
one single plan. However, each plan has to be consistent with other plans to have a viable business
continuity planning strategy. It may not be possible to define a sequence in which plans have to be
implemented, as it may be dependent on the nature of disaster, criticality, recovery time, etc.

One of the main objectives of an audit is to identify potential risks; therefore, the most proactive approach would be to identify and evaluate the existing security practices being followed by the organization. IS auditors should not prepare documentation, as doing so could jeopardize their independence. Terminating the audit may prevent achieving one of the basic audit objectives, i.e., identification of potential risks. Since there are no documented procedures, there is no basis against whichto test compliance.