Final answer:
The best option for the CAE to conduct the urgent forensic investigation is to outsource the investigation to independent professional consultants.
Step-by-step explanation:
The chief audit executive's best option for conducting an ad hoc forensic investigation of the purchasing department is (A). Outsource the investigation to independent professional consultants. Given the significance, urgency, and the lack of available staff with relevant experience, outsourcing to qualified professionals ensures that the investigation is conducted effectively and within the required time frame.
Option B is not viable due to the time-sensitive nature of the engagement, as training internal auditors would take too much time. Recruiting additional staff with the required skills (Option C) may also be time-consuming and there's no guarantee that new hires could be made quickly enough to meet the deadline. Option D, declining the engagement, is not advisable because it fails to address the immediate risk identified by the audit committee.