Option C is the best first course of action because, in a complex intrusion involving anti-forensics , the investigation must first establish the scope and impact of the breach before deeper reverse engineering or broad remediation decisions are made. CHFI v11 emphasizes the forensic investigation process , including first response , evidence preservation , case analysis , and understanding indicators of compromise and anti-forensics challenges .
Determining exactly what data was compromised is foundational. It helps investigators define the severity of the incident, identify affected systems and stakeholders, prioritize evidence collection, and support legal, regulatory, and business response requirements. Without first establishing the compromised data set, later activities such as reverse engineering attacker methods or deploying broad controls may be less targeted and less effective.
Option A may become important later, but it is not the most immediate first step in understanding breach impact. B and D are response measures, yet prematurely changing the environment can complicate forensic analysis. Therefore, under CHFI's investigation-process and anti-forensics principles, the most appropriate first action is to identify the exact data that has been compromised .