Choice "C" is correct. Inquiry, observation, inspection, recalculation, and reperformance are procedures used to test operating effectiveness of internal controls.Choice "a" is incorrect. Mailing confirmations is a substantive procedure, which provides evidence about the dollar balance of an account.Choice "d" is incorrect. Analytical procedures do not provide evidence about the operating effectiveness of a control. However, analytical procedures may be used in the testing phase of the audit as a substantive procedure to provide evidence about the dollar balance of an account.Choice "b" is incorrect. Recalculation related to the balance of accounts is a substantive procedure. Recalculation related to a control would help the auditor obtain evidence of the control's effectiveness.