Explanation: An MMSI (Maritime Mobile Service Identity) is a unique 9-digit number assigned to a vessel's Digital Selective Calling (DSC) radio, functioning much like a phone number for maritime communications. While it's critical for emergency services to have a stable identifier, the ability to change an MMSI number varies significantly by VHF DSC radio manufacturer and model. Some modern VHF DSC units are designed to allow the operator to reprogram the MMSI number multiple times, provided they know the specific password set for the unit's programming functions. This flexibility can be useful if a radio is transferred to a new vessel. However, other units, particularly older or more basic models, may lock the MMSI after initial entry, requiring a service center or even the manufacturer to change it, or they might be permanently locked. Therefore, "On some units it can be changed at will if the password is known" (D) is the most accurate statement, acknowledging the variation in capabilities across different equipment. Options A and C are incorrect because changes *are* possible on many units, not solely by the manufacturer. Option B suggests a specific "4 times" limit which is not a universal rule.