Answer: D. At least every third audit
Explanation: According to the AICPA Professional Standards of auditing. Testing the operating effectiveness of controls that appear to function as they have in past years and on which the auditor wishes to rely upon in the current year must be carried out at least every 3rd Audit.
This is done if the auditor plans to use audit evidence from a prior audit about the operating
effectiveness of specific controls. The auditor shall establish the continuing relevance of that
evidence by obtaining audit evidence about whether changes in those controls have
occurred subsequent to the previous audit.