The correct choice is C because preserving original evidence is one of the most fundamental rules of forensic acquisition. CHFI v11 places strong emphasis on evidence preservation, proper acquisition methodology, chain of custody, and validation of collected data. The core idea is that the original source should remain unchanged so that the examiner can later demonstrate integrity, repeatability, and legal defensibility. During acquisition, investigators typically work from forensic copies and use write-protection wherever possible precisely to avoid altering the original evidence. Although documenting every process is also extremely important, documentation supports admissibility; it does not replace the primary requirement to preserve the original evidence in an unmodified state. Quality assurance and reduced exposure are helpful principles, but they are not the central rule of thumb that governs acquisition integrity. In a courtroom or formal investigation, one of the first questions is whether the original evidence was preserved without contamination or modification. That is why CHFI repeatedly ties acquisition best practices to preservation and validation. For exam purposes, when the question asks for the main rule of thumb during acquisition, preserve original evidence is the most defensible answer.