The best way to provide assurance of the integrity of a firewall log is to ensure that the log cannot be modified. A firewall log is a record of the traffic and events that occur at the firewall, which is a device or software that controls and filters the incoming and outgoing network traffic based on predefined rules and policies. The integrity of a firewall log means that the log is accurate, complete, consistent, and valid, and that it has not been altered, deleted, or corrupted by unauthorized or malicious parties. The IS auditor should verify that the firewall log has adequate controls to prevent or detect any modification of the log, such as encryption, hashing, digital signatures, write-once media, or tamper-evident seals. The other options are not as effective as ensuring that the log cannot be modified, because they either do not address the integrity of the log data, or they are monitoring or retention measures rather than preventive or detective controls. References: CISA Review Manual (Digital Version)1, Chapter 5, Section 5.2.4

If an IS auditor suspects that independence may not have been maintained in past audits, the best course of action is to inform audit management. Audit management has the responsibility and authority to address such issues. They can review the situation, determine if there was indeed a lack of independence, and decide on the appropriate actions to take123. While informing senior management, reevaluating internal controls, and re- performing past audits might be necessary at some point, the first step should be to inform audit management.

The greatest concern with the IT department and internal IS audit function both reporting to the CIO is the potential compromise of IS audit independence. Auditor independence refers to the impartiality and objectivity of an auditor in conducting an audit, free from conflicts of interest and bias1. It is crucial for ensuring the quality and reliability of financial reporting1. If the IS audit function reports to the CIO, who also oversees the IT department, it could create a conflict of interest that might compromise the impartiality and objectivity of the IS audit function.
References:
Auditor Independence - What is it, Rules, Importance, Examples