Choice "A" is correct. By using statistical sampling, the auditor can quantify sampling risk to assist in limiting it to a level considered acceptable.
Choice "c" is incorrect. Statistical sampling does not provide any advantage with respect to converting the test into a dual-purpose test.
Choice "d" is incorrect. Statistical sampling still requires judgment to determine sample sizes. The tolerable rate of deviation, the likely rate of deviation, and the allowable risk of assessing control risk too low are all determined by the auditor's professional judgment.
Choice "b" is incorrect. Statistical sampling does not afford greater assurance than a nonstatistical sample of the same size. It only provides the auditor with a better measure of the sufficiency of the evidence found, and helps to evaluate the results found.