Choice "D" is correct.  Statements of Quality Control Standards (SQCS) are issued to provide guidance with respect to audit quality control. These standards indicate that the nature and extent of a firm's quality control policies and procedures are directly tied to its organizational structure. So by maintaining a comprehensive system of quality control in relation to its organizational structure, the CPA firm would provide reasonable assurance that its professional services conform to the above standards. Choice "a" is incorrect. Specific individual responsibilities are not outlined in an engagement letter.  Furthermore, the engagement letter does not provide reasonable assurance that the audit firm is meeting its responsibility to provide professional services that comply with professional standards. This is done through a system of quality control.Choice "b" is incorrect. Assessing the risk of material misstatement is part of an audit engagement and does not provide reasonable assurance that the audit firm is meeting its responsibility to provide professional services that comply with professional standards. This is done through a system of quality control.Choice "c" is incorrect.  Developing specific audit objectives to support management's assertions in the financial statements is an audit procedure that does not provide reasonable assurance that the audit firm is meeting its responsibility to provide professional services that comply with professional standards. This is done through a system of quality control.