Option D is the best answer because the scenario strongly suggests a deceptive email-based social engineering event that triggered the movement and disappearance of the files. The most important next step is to identify whether the email was genuinely sent by the manager or whether it was spoofed, relayed, or otherwise malicious. In CHFI terms, this aligns with examining the source of the incident , correlating email artifacts with server logs , and reconstructing the sequence of events to determine attribution and attack method.

Although disk images have already been acquired, the question asks for the next step in the investigation. Since the trigger was a suspicious instruction sent by email, analyzing email headers can reveal sender path, relay details, spoofing indicators, and authentication issues. Reviewing server logs can then confirm access activity, file movement, and related actions around the same timeframe. That gives the investigator a clearer understanding of whether this was phishing, impersonation, or insider misuse.

Option A is too broad, B is premature, and C focuses on consequences rather than the initiating cause. Therefore, header and log analysis is the strongest first analytical move.