Choice "D" is correct. The ultimate purpose of assessing control risk is to contribute to the auditor's evaluation of the risk that material misstatements exist in the financial statements.
Choice "c" is incorrect. Sometimes risk assessment procedures may raise doubts about the auditability of the financial statements, but this is not the ultimate purpose of assessing control risk.
Choice "a" is incorrect. The auditor is required to obtain knowledge about the operating effectiveness of internal controls, if his or her risk assessment is based on the effective operation of controls. Obtaining such knowledge, however, is not the ultimate purpose of assessing control risk.
Choice "b" is incorrect. Although the nature and extent of substantive tests may be reduced based on the results of the control risk assessment, this is not the ultimate purpose of assessing control risk.