Final answer:
An auditor would most likely use the entity's annualized interim financial statements in determining the auditor's planning materiality.
Step-by-step explanation:
An auditor would most likely use the entity's annualized interim (i.e. quarterly) financial statements in determining the auditor's planning materiality. The auditor can analyze the financial statements to assess the overall financial performance and risk of the entity. Materiality is the threshold used to determine the significance of errors or misstatements in the financial statements.
By reviewing the financial statements, the auditor can identify key accounts and balances that are important to the financial health of the entity. This helps in setting the materiality level for planning the audit procedures. The financial statements provide the necessary information for the auditors to evaluate the materiality of potential misstatements and plan their audit procedures accordingly.
Other options mentioned in the question, such as the anticipated sample size for planned substantive procedures, the results of the internal control questionnaire, and the contents of the management representation letter, may be used in different stages of the audit but are not directly related to determining the auditor's planning materiality.