If the assignment is a consulting engagement, the best action for the new internal auditor, who recently transferred from being an accounts payable clerk, is to decline the assignment and ask to be reassigned. This avoids any conflict of interest and maintains objectivity, as the auditor would be evaluating processes for which they were previously responsible, potentially compromising the independence and objectivity required in consulting engagements.
IIA Standards 1120 - Objectivity

Fraud involving significant amounts, such as theft using collusion for more than $10,000, should be reported to the audit committee at the next meeting. This type of fraud indicates a higher level of risk and potential impact on the organization, which makes it critical for the audit committee to be informed promptly so that appropriate measures can be taken.
Guidelines on fraud reporting from the Institute of Internal Auditors

The most effective fraud prevention control among the listed options is an email alert sent to management for checks issued over $100,000. This control directly addresses a potential high-risk area (large transactions) and ensures that transactions of significant amounts are reviewed and approved by management, thus providing a strong deterrent and detection mechanism for fraudulent activity.
Common financial control practices and fraud prevention mechanisms in financial management.

One of the possible reasons for the failure to identify transactions between the parent organization and a subsidiary is that the auditor did not have sufficient knowledge of the group structure, the consolidation process, and the related party disclosure requirements. The auditor should obtain an understanding of the entity and its environment, including its internal control, as part of the risk assessment procedures.

In a consulting engagement regarding a debt collections process, an internal auditor would primarily focus on advising management on streamlining the recording of accounts receivable. This action aims to add value by providing expert advice to improve the efficiency and effectiveness of the process, consistent with the IIA's guidelines on the role of internal auditors in consulting activities.
IIA Standard 1000 - Purpose, Authority, and Responsibility