Which document is critical for outlining the auditor's responsibilities and the scope of the audit?

The engagement letter serves a vital function in the audit process by clearly defining the responsibilities of the auditor and the scope of the audit. It establishes the terms under which the audit will be conducted, including the objectives, deliverables, and timelines, which helps ensure that both the auditor and the client have a mutual understanding of what to expect from the audit engagement.

By detailing the auditor's responsibilities, the engagement letter helps to prevent misunderstandings and sets a foundation for accountability. It also outlines any specific areas of focus, helping to ensure that the audit aligns with the organization’s needs and expectations. In addition, this document can specify any limitations on the audit work, guiding the auditor on what aspects may not be included within the engagement.

The other documents mentioned, while important in their own right, do not specifically fulfill this role. The audit report primarily summarizes findings and recommendations after the audit has been conducted, the risk assessment plan involves identifying and evaluating risks but doesn't describe the auditor’s role directly, and the non-disclosure agreement focuses on confidentiality rather than the specifics of the audit engagement.