Choice "D" is correct. The factor most likely to cause the auditor to promptly apply the omitted procedure would be if the omission impairs the auditor's present ability to support the previously expressed opinion.
Choice "b" is incorrect. The auditor would first need to determine whether the omission of the procedure impairs the ability to support the previously expressed opinion. It is possible that other procedures already performed tended to compensate for the omitted procedure, and therefore no further action would be necessary.
Choice "a" is incorrect. The availability of the source documents needed to perform the procedure has little bearing on whether the auditor determines that it is necessary to perform the procedure.
Choice "c" is incorrect. The rendering of a clean opinion has little importance. The issue is "support of the previously expressed opinion," not what the opinion was.