The correct answer is D because IMSI is the identifier tied to the subscriber identity in the mobile network. GSMA explains that the IMSI uniquely identifies the subscription associated with a SIM and is used by the mobile network to recognize the subscriber. That makes it the best field when investigators want to correlate tower activity and account-level records to the subscriber rather than to the handset hardware. By contrast, IMEI identifies the physical device, MSISDN is the dialable phone number, and Cell ID identifies the serving cell tower sector rather than the user account. CHFI v11 includes cellular network components, subscriber identity modules, and cell site analysis, so candidates are expected to distinguish subscriber identity from device identity and location identifiers. In a forensic context, if the goal is to associate movements across towers with the actual subscription record held by the carrier, the most relevant field is IMSI. That is the core subscriber identifier used by the provider’s network systems. (gsma.com)