Section: The process of Auditing Information System

The best answer is B.
A quality assurance and improvement program is meant to evaluate whether the audit function is performing effectively and independently. ISACA's candidate guide explicitly includes evaluating audit processes as part of QA and improvement programs, and ISACA peer-review guidance states that a peer review process assessing internal audit is essential to ensure it is appropriately carrying out its role. The absence of independent periodic external review is therefore the most serious concern because it weakens objective assessment of audit quality itself.
Option A is a weakness, but tracking improvement opportunities centrally is secondary to having independent validation that the function meets expected quality standards. Option C is not the best answer because substantive testing during an assessment phase may vary by engagement design. Option D matters for governance communication, but it is still less central to QA than the lack of external independent review.
References (Official ISACA):
* ISACA, Certification Exam Candidate Guide.
* ISACA Journal, A Standardized Approach for Peer Review of Internal Audit.