Which of the following is required in writing according to ISA 210?

Management's responsibilities must be documented in writing according to ISA 210, which deals with the agreement on the terms of audit engagements. This standard emphasizes the necessity of a clear understanding between the auditor and the entity about the responsibilities of the management. This written documentation helps to confirm that management acknowledges its responsibilities for the preparation of the financial statements and internal controls, as well as adherence to applicable laws and regulations. This clarity is crucial for ensuring that both parties have a mutual understanding of the expectations and obligations associated with the audit.

While other options might involve aspects of the audit process, they do not have the same explicit requirement under ISA 210 for a written agreement. For instance, an auditor's personal opinion is subjective and not a formal requirement, audit cost estimation is generally discussed but not mandated to be documented in writing within ISA 210, and the names of external audit team members are typically communicated but not required as part of the engagement letter. Thus, the focus on articulating management's responsibilities in writing serves to protect both the auditor and the client by establishing a shared understanding that is essential for the effective conduct of the audit.