Choice "D" is correct. If the auditor determines that testing the operating effectiveness of controls would not be efficient, he or she may choose to use only substantive procedures to evaluate specific assertions and risks. Choice "c" is incorrect. The fact that internal control components are not well documented does not necessarily mean that the auditor would use only substantive procedures. A primarily substantive approach is taken when there are no effective controls, when implemented controls are assessed as ineffective, or when it would not be efficient to test controls. Choice "b" is incorrect.  Although the internal auditor may assist with certain aspects of the audit, the judgment of the internal auditor regarding the effectiveness of the controls cannot be used by the external auditor.Choice "a" is incorrect. The auditor will generally take a combined approach when it is efficient or cost effective to test controls.