Final answer:
The auditor is testing D) Application controls. Application controls are specific to the processing of transactions within computer programs. The use of test data is a method to verify the accuracy and functioning of these controls.
Step-by-step explanation:
If an auditor is using test data in a client's computer system to test the integrity of the system's output, the type of controls the auditor is testing is D) Application controls. Application controls are specific to computer programs and the processing of transactions. They ensure the completeness, accuracy, and validity of the data that is being processed. By inserting test data into the system, the auditor can assess whether the application is functioning correctly and producing accurate outputs.
In contrast, General controls refer to the overall framework within which applications operate. They include controls over data center operations, system software acquisition and maintenance, and access security. User controls are procedures that individual users are responsible for to ensure correctness of their transactions and activities. Quantitative test controls are not a recognized category in auditing computer system controls.
It's important to note that the role of experimental controls mentioned in the provided information does not directly apply to auditing computer systems but rather to scientific experimentation where controls help to ensure a test's validity. In the context of auditing, the test data approach more closely aligns with verifying application controls.