In internal auditing, it is crucial to have the necessary skills and expertise to effectively evaluate and assess the area under review. When internal auditors lack the required skills, the chief audit executive (CAE) must take steps to ensure the engagement is performed effectively. Option D suggests bringing in a consultant who is independent of the area under review and has the missing expertise. This approach ensures that the engagement is conducted with the necessary knowledge and objectivity, thus maintaining the quality and integrity of the audit process. Unlike options A and B, which compromise the audit's effectiveness, and option C, which could create conflicts of interest, option D provides an optimal solution.
Reference: The IIA's International Standards for the Professional Practice of Internal Auditing, Standard 1210 - Proficiency and Standard 1220 - Due Professional Care.