The best way to ensure a successful investigation is to preserve digital evidence immediately, including both a forensic image of the system and volatile memory capture where feasible. ISACA defines digital forensics as identifying, preserving, analyzing, and presenting digital evidence in a legally acceptable manner. That makes evidence preservation the first priority.

Option A is correct because creating a system image preserves the state of the disk, while capturing memory preserves volatile information that may be lost if the system is altered, powered off, or rebooted. ISACA’s glossary notes that memory dumps allow analysis of memory contents at the time of failure, which supports preserving volatile evidence for forensic review.

Option B is not the best answer. Isolating the system may help preserve evidence, but by itself it does not collect the volatile and nonvolatile evidence needed for a strong investigation. Evidence preservation is stronger when the system is forensically imaged and memory is captured.

Option C is incorrect because rebooting can destroy volatile evidence and alter timestamps or system state, which can undermine a forensic investigation. That runs directly counter to digital forensics principles centered on preservation.

Option D may be useful later, but interviews are secondary to preserving digital evidence. Human recollection cannot substitute for properly preserved forensic artifacts.

Therefore, A is the best answer because it preserves the most complete and reliable digital evidence for investigation.

References (Official ISACA):

ISACA, Overview of Digital Forensics — digital forensics is the process of identifying, preserving, analyzing and presenting digital evidence.

ISACA Glossary, Digital forensics.

ISACA Glossary, Memory dump.