Comprehensive and Detailed Explanation:

Even though this is an advisory engagement, internal auditors must still evaluate significant control gaps discovered during the engagement (Standard 2120 – Risk Management). Since a key procurement control is missing, the auditor should not ignore the risk (D). Expanding the advisory engagement scope (C) would be outside the engagement mandate unless approved. Deferring review to next year’s plan (A) delays needed assurance. The most appropriate step is to perform a root cause analysis and test whether the approved workaround is effective (B). This ensures that risks are mitigated in the interim until a permanent control is implemented, providing management with actionable feedback.