The most reliable evidence of a control's effectiveness is a system-generated testing report. A system-generated testing report is a document that shows the results of automated tests performed by the system to verify that the control is functioning as intended and producing the expected outcomes. A system-generated testing report is reliable, because it is objective, consistent, accurate, and timely, and because it can provide a high level of assurance and confidence in the control's effectiveness. The other options are not as reliable as a system-generated testing report, although they may provide some evidence of the control's effectiveness. A risk and control self-assessment, senior management's attestation, and a detailed process walk-through are all examples of manual or subjective evidence, which may be prone to errors, biases, or inconsistencies, and which may provide a lower level of assurance and confidence in the control's effectiveness. References = Risk and Information Systems Control Study Manual, Chapter 3, Section 3.4.1, page 3-32.