Choice "A" is correct. The quantity, type, and content of audit documentation vary with the circumstances, but it should be sufficient to show that the accounting records agree or reconcile with the financial statements.
Choice "d" is incorrect. The auditor's understanding of internal control can be documented in a variety of different ways. Flowcharts and internal control questionnaires are two methods that might be used to assist the auditor in obtaining an understanding of internal control, but they generally do not provide information about the effectiveness of the entity's internal controls.
Choice "b" is incorrect. An engagement letter (and not a representation letter) summarizes the timing and details of the auditor's planned field work.
Choice "c" is incorrect. A list of procedures and findings is required for an agreed-upon procedures engagement, not for an audit.