Choice "C" is correct. The test data approach refers to a technique in which the client's application program is used to process a set of test data, the results of which are already known by the auditor. If the client's program is operating effectively, it should generate the same results determined by the auditor.
Choice "a" is incorrect. An integrated test facility (not a test data approach) utilizes dummy accounts. For example, using an integrated test facility (ITF) approach, test data is initially commingled with live data, but coding to a dummy account allows later extraction from the system under actual operating conditions.
Choice "b" is incorrect. Test data programs should be tailor-made by the auditor for each client's computer applications, to ensure that the data is in an appropriate form for that client's system, and that it includes the types of invalid conditions in which the auditor is interested.
Choice "d" is incorrect. Test data contains the types of valid and invalid conditions in which the auditor is interested (it is not necessary to test all combinations of valid and invalid conditions).