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D is corrent because when internal auditors only report to a relatively low level in the organization (here the controller and not, for example, the audit committee), the independent auditor will question the internal auditors’ ability to remain objective and this will result in limited reliance being placed upon their work.
A is incorrect because auditors assess control risk, they do not reduce control risk--control risk is a function of the client’s internal control structure. Also, this answer has only an indirect relationship to the topic of the question, that of using internal auditors to provide assistance in performing the audit.
B is incorrect because with proper supervision, the independent auditors may choose to use the work of internal auditors.
C is incorrect because tests of controls do not relate directly to detection risk and because the independent auditors do not assess detection risk (they perform substantive tests to restrict it).