Choice "C" is correct. Audit documentation, or working papers, comprises the principal record of audit procedures performed, evidence obtained, and conclusions reached. Reviewing the working papers allows a supervisor to understand the work performed and the evidence obtained, and to evaluate whether the audit was adequately performed.
Choice "d" is incorrect. Checklists might be used within the audit documentation, but checklists alone would not provide a comprehensive record of the audit procedures performed, the evidence obtained, and conclusions reached.
Choice "b" is incorrect. Analytical procedures might be documented within the working papers, but such procedures alone would not provide a comprehensive record of the audit procedures performed, the evidence obtained, and conclusions reached.
Choice "a" is incorrect. Reviewing the financial statements would provide no information regarding the audit procedures performed, the evidence obtained, or conclusions reached, and therefore would provide no basis on which to review the work performed by the staff.