Choice "C" is correct. As the acceptable level of detection risk increases, the assurance (effectiveness) that must be provided from substantive tests can decrease. Thus, the auditor may change the timing of substantive tests from year-end to an interim date.
Choice "d" is incorrect. The assessed level of control risk is used to determine the acceptable level of detection risk, not vice versa.
Choice "a" is incorrect. The acceptable level of detection risk affects the nature, timing and extent of substantive audit procedures, not control tests.
Choice "b" is incorrect. As the acceptable level of detection risk increases, the level of assurance required from substantive tests decreases, and therefore the substantive procedures need not be as effective.