Explanation
The audit charter is the document that defines the purpose, authority and responsibility of the IS audit function. It provides IS audit professionals with the best source of direction for performing audit functions, as it establishes the scope, objectives, reporting lines, independence, accountability and resources of the IS audit function. The IT steering committee is a governance body that oversees the strategic alignment, prioritization and direction of IT initiatives, but it does not provide specific guidance for IS audit functions. The information security policy is a document that defines the rules and principles for protecting information assets in the organization, but it does not cover all aspects of IS audit functions. Audit best practices are general guidelines and recommendations for conducting effective and efficient audits, but they are not binding or authoritative sources of direction for IS audit functions. References: CISA Review Manual (Digital Version) 1, Chapter 1:
Information Systems Auditing Process, Section 1.1: Audit Charter.