Explanation
The rules of engagement define the scope, objectives, methodology, deliverables, and limitations of the penetration testing. They also specify the legal and ethical boundaries, communication channels, and escalation procedures. Establishing the rules of engagement is the first step when planning to conduct penetration testing for a client, as it ensures that both parties agree on the expectations and outcomes of the testing. The other options are important steps, but they should be done after the rules of engagement are established. References: CISA Review Manual (Digital Version) 1, page 381.

The type of review that is most important to conduct when an IS auditor is informed that a recent internal exploitation of a bug has been discovered in a business application is C. Forensic audit. A forensic audit is a type ofaudit that involves collecting, analyzing, and preserving evidence of fraud, corruption, or other illegal or unethical activities1. A forensic audit can help the IS auditor to identify and document the source, scope, and impact of the exploitation, as well as the perpetrators, motives, and methods involved. A forensic audit can also help the IS auditor to provide recommendations for preventing or mitigating future exploitations, and to support any legal actions or investigations that may arise from the incident2.