According to The IIA's Code of Ethics, the principle of integrity emphasizes the importance of honesty and fairness in the auditor's conduct. The statement that best reflects this principle is that auditors must disclose all material facts known to them that, if not disclosed, may distort the reporting of activities under review. This aspect of integrity ensures transparency and accuracy in the presentation of audit findings, crucial for the credibility of the audit function and the trust placed in it by stakeholders.
The IIA's Code of Ethics - Integrity

The source of authority for an engagement supervisor to make contact with external parties, such as obtaining maintenance reports from a contractor, typically comes from the provisions outlined in the internal audit charter. This charter formally defines the purpose, authority, and responsibility of the internal audit activity, including interactions with third-party service providers. It is essential as it sets the audit activity's scope, allowing auditors to access necessary information and resources.
The Institute of Internal Auditors (IIA) - International Professional Practices Framework (IPPF), specifically the Audit Charter guidelines.

Input controls are designed to ensure the accuracy, completeness, and validity of data entered into a business application. These controls can include validation checks, input masks, and error detection methods that verify data at the point of entry. Whether data is entered directly by staff, remotely by business partners, or through web-enabled applications, input controls help maintain the integrity of the data by preventing errors and unauthorized input. These controls are crucial in maintaining data quality and integrity in any business application.
The IIA's Global Technology Audit Guide (GTAG) on Information Technology Controls.
COBIT 5 Framework on Information and Technology Governance.

Facilitating internal auditors in upholding their objectivity within the internal audit manual can effectively be addressed by ensuring that internal auditors regularly attend professional workshops to refresh and update their understanding of internal audit norms and concepts. This practice helps maintain a high level of professionalism and objectivity by keeping auditors informed about the latest standards and ethical guidelines, which in turn minimizes the risk of biases and enhances their ability to perform independent and objective audits.
IIA's International Standards for the Professional Practice of Internal Auditing on Continuing Professional Development.

The most appropriate first step for the board to take when developing an effective system of governance is to identify key stakeholders and their expectations. Understanding stakeholders' expectations is fundamental to defining the governance framework that aligns with these needs and establishing the organization's strategic objectives and policies.
IIA guidance on effective governance frameworks.