In a mature control environment with limited internal audit resources, internal auditors should focus their testing on preventive key controls. Preventive controls are designed to stop errors or irregularities before they occur, making them crucial for maintaining control effectiveness. Key controls are the most important controls in mitigating risks to an acceptable level. By focusing on these, internal auditors can ensure that the most critical risks are managed effectively despite limited resources. References:
* The IIA's International Standards for the Professional Practice of Internal Auditing (Standards), specifically Standard 2320 - Analysis and Evaluation.
* The IIA's Practice Guide on Assessing the Adequacy of Control Processes.

Comprehensive and Detailed Explanation:
Per IIA Standard 1210 - Proficiency, engagements must be performed by auditors with the necessary skills. If resources are not available, the engagement supervisor cannot unilaterally suspend or reassign staff without CAE input. The appropriate action is to escalate the issue to the CAE (D), who has authority to adjust the audit plan, bring in external expertise, or reallocate resources. Option A may not be feasible due to scheduling conflicts. Options B and C prematurely halt the engagement, which undermines assurance delivery. The CAE is responsible for ensuring resource adequacy (per Standard 2030 - Resource Management). Therefore, the engagement supervisor must seek guidance from the CAE.

Sampling is commonly involved in reperformance and inspection procedures. Reperformance involves the internal auditor independently executing procedures or controls to verify the results obtained by the entity.
Inspection involves examining records, documents, or tangible assets. Both of these audit procedures frequently use sampling techniques to select items for testing, which helps ensure that the auditor's conclusions are based on representative data without the need to examine every item in the population.
The Institute of Internal Auditors (IIA) Practice Guide on "Audit Sampling" Generally Accepted Auditing Standards (GAAS) related to audit evidence and sampling

In the risk control map, Risk C is positioned in the upper left quadrant, indicating it is critical (high risk significance) but with a low level of control. This suggests that the current controls are insufficient to mitigate the high level of risk associated with Risk C. For critical risks, a higher level of control is necessary to ensure that the risk is properly managed and mitigated. References:
* "Internal Auditing: Assurance & Advisory Services" (The Institute of Internal Auditors)
* "Risk Management Framework" (COSO)

According to IIA guidance, the chief audit executive (CAE) is responsible for evaluating and verifying management's response to audit recommendations. The CAE must also determine the need and scope for additional work based on the adequacy and timeliness of management's corrective actions. This ensures that the audit recommendations are effectively implemented and that any residual risks are appropriately managed.
References: IIA Standard 2500 - Monitoring Progress, IIA Practice Advisory 2500.A1-1