The best course of action for an IS auditor when an auditee is unable to close all audit recommendations by the time of the follow-up audit is to evaluate the residual risk due to open issues. Residual risk is the risk that remains after the implementation of controls or mitigating actions. Evaluating the residual risk due to open issues can help the IS auditor assess the impact and likelihood of the potential threats and vulnerabilities that have not been addressed by the auditee, as well as the adequacy and effectiveness of the existing controls or mitigating actions. Evaluating the residual risk due to open issues can also help the IS auditor prioritize and communicate the open issues to the auditee and other stakeholders, such as senior management or audit committee, and recommend appropriate actions or escalation procedures.

Ensuring the open issues are retained in the audit results is a course of action for an IS auditor when an auditee is unable to close all audit recommendations by the time of the follow-up audit, but it is not the best one. Ensuring the open issues are retained in the audit results can help the IS auditor document and report the status and progress of the audit recommendations, as well as provide a basis for future follow-up audits. However, ensuring the open issues are retained in the audit results does not provide an analysis or evaluation of the residual risk due to open issues, which is more important for informing decision-making and action-taking.

Terminating the follow-up because open issues are not resolved is not a course of action for an IS auditor when an auditee is unable to close all audit recommendations by the time of the follow-up audit, but rather a consequence or outcome of it. Terminating the follow-up because open issues are not resolved may indicate that the auditee has failed to comply with the agreed-upon actions or deadlines, or that the IS auditor has encountered significant obstacles or resistance from the auditee. Terminating the follow-up because open issues are not resolved may also trigger further actions or sanctions from the IS auditor or other authorities, such as issuing a qualified or adverse opinion, withholding certification, or imposing penalties.

Recommending compensating controls for open issues is not a course of action for an IS auditor when an auditee is unable to close all audit recommendations by the time of the follow-up audit, but rather a possible outcome or result of it. Compensating controls are alternative or additional controls that are implemented to reduce or eliminate the risk associated with a weakness or deficiency in another control. Recommending compensating controls for open issues may be appropriate when the auditee is unable to implement the original audit recommendations due to technical, operational,financial, or other constraints, and when the compensating controls can provide a similar or equivalent level of assurance. However, recommending compensating controls for open issues requires a prior evaluation of the residual risk due to open issues, which is more important for determining whether compensating controls are necessary and feasible.