The correct answer is: Confirm or revise an earlier estimate of the strength or otherwise of internal controls.

Tests of controls are performed to obtain audit evidence about the effectiveness of the design and operation of the internal controls.

Estimating the likely rate of occurrence of errors in an accounting system is not a purpose of controls testing. This is achieved through substantive procedures.

Tests of controls do not help to form an initial judgement as to the strength or otherwise of a system of internal control, as this should have already been done at the planning stage.

Where significant deficiencies in internal control are identified as part of controls testing, these will be reported to management. While this may be a result of tests of control, this is not the main purpose of performing the procedure.