Explanation
The first action that an IS auditor should take when finding a high-risk vulnerability in a public-facing web server used to process online customer payments is to identify compensating controls. Compensating controls are alternative or additional controls that provide reasonable assurance of mitigating the risk of exploiting the vulnerability. The IS auditor should assess the effectiveness of the compensating controls and determine whether they reduce the risk to an acceptable level. If not, the IS auditor should recommend remediation actions to address the vulnerability. Documenting the exception in an audit report is an important action, but it should not be the first action, as it does not address the urgency of the situation. Reviewing security incident reports is a useful action, but it should not be the first action, as it does not provide assurance of preventing future incidents. Notifying the audit committee is a necessary action, but it should not be the first action, as it does not involve taking any corrective measures. References:
CISA Review Manual, 27th Edition, pages 295-2961
CISA Review Questions, Answers & Explanations Database, Question ID: 260

The best answer is C. Re-performance of controls.
Under ISACA audit principles, evidence obtained directly by the auditor is generally more reliable than evidence provided by management or gathered indirectly. Re-performance allows the auditor to independently execute the control or procedure and verify whether it works as intended, making it stronger than inquiry, observation, or management attestation.
Option A is the least reliable because inquiry depends on what people say. Option B is stronger than simple inquiry but still relies on management representation. Option D can be useful, but observation only shows what happened at a point in time and may not prove consistent operation. Re-performance gives the auditor the highest level of assurance because the evidence is generated through the auditor's own independent work.
References (Official ISACA):
ISACA, Follow-Up Audits and Follow-Up Process: The Auditor's Impact Litmus Tool ISACA, The Top-Five Audit Essentials for Driving Efficiency and Value