Choice "A" is correct. Test data allows the auditor to determine whether adequate controls exist over data processing. Test data consists of fictitious entries or inputs that are processed through the client's computer system under the control of the auditor. The client's computerized payroll system should have adequate controls to prevent input of invalid employee I.D. numbers.
Choice "c" is incorrect. This control does not involve the client's computer system and therefore cannot be tested using test data.
Choice "b" is incorrect. This control does not involve the client's computer system and therefore cannot be tested using test data.
Choice "d" is incorrect. This control does not involve the client's computer system and therefore cannot be tested using test data.