When an internal auditor suspects fraudulent activity, such as erroneous payments to a fraudulent bank account, the appropriate course of action is to escalate the concern to the engagement supervisor (Option D).
This step ensures that the issue is handled with the necessary urgency and oversight. According to the IIA Standards, particularly Standard 2060: Reporting to Senior Management and the Board, the CAE must communicate significant risk exposures and control issues, including fraud risks, to senior management and the board. Escalating the concern ensures the appropriate levels of the organization are aware and can take timely action.References:
* IIA Standards, Standard 2060: Reporting to Senior Management and the Board
* IIA Practice Guide: Internal Auditing and Fraud

Critical thinking skills are essential for internal auditors when designing audit procedures and selecting samples. The situation described indicates that the auditor relied on a sampling method without thoroughly considering the potential risks and the specific context of the transaction population. Improved critical thinking skills would help the auditor better assess the representativeness of the sample, identify potential fraud risks, and ensure a more thorough and effective audit approach, potentially preventing the oversight that led to undetected fraud.
References:
* The IIA Standards: Standard 1220 - Due Professional Care: "Internal auditors must apply the care and skill expected of a reasonably prudent and competent internal auditor. Due professional care includes consideration of the use of technology-based audit and other data analysis techniques."
* IIA Practice Guide: "Assessing the Adequacy of Risk Management and Controls": Emphasizes the importance of critical thinking in evaluating risks and controls.

According to IIA guidance, the internal audit charter should formally document the nature of consulting services provided by the internal audit activity. This ensures that the scope and extent of consulting services are clearly defined and understood, helping to manage expectations and clarify the role of the internal audit function in such activities.References: IIA's International Professional Practices Framework (IPPF) - specifically, the attribute standards relating to the internal audit charter which should outline the nature of consulting services among other fundamental roles and responsibilities.

Nonconformance with the Standards that requires disclosure to the board includes the failure to conduct external assessments within the timeframe stipulated by the IIA standards. Specifically, Standard 1312 requires that an external quality assessment must be conducted at least once every five years. Not completing this assessment within the last seven years constitutes a nonconformance that must be reported.References:
Institute of Internal Auditors (IIA) Standards, specifically Standard 1312 on External Assessments.

An impairment to independence occurs when the CAE's supervisor is responsible for functions that the CAE may need to audit. This dual responsibility can create a conflict of interest and impact the objectivity of the internal audit activity. References:
* IIA Standard 1110: Organizational Independence.
* IIA Practice Guide: Independence and Objectivity.