The chief audit executive (CAE) is responsible for the approval of the engagement objectives and scope. While the head of customer service and other stakeholders may provide input, it is ultimately the CAE's responsibility to ensure that the engagement aligns with the internal audit plan and meets the organization's overall objectives. The CAE's approval ensures the independence and objectivity of the internal audit function.
Reference:
The Institute of Internal Auditors (IIA) Standard 2010 - Planning
IIA Standard 2200 - Engagement Planning