Explanation: In the context of marine radio communications, particularly for Digital Selective Calling (DSC) systems, an address book functions as a contact list. The **MMSI (Maritime Mobile Service Identity) number** is the unique nine-digit digital identifier for a vessel, analogous to a phone number. It is essential for directly initiating a DSC call to another vessel. The **vessel's name** is typically entered alongside the MMSI to provide a human-readable identifier, making it easier to select the correct contact and verify the identity of the station being called. * **A) The vessel’s IMN (IMO Number):** While a unique identifier, the IMO number is primarily for administrative and safety purposes, not directly used as a radio "address" in an address book. * **B) The vessel’s call sign and name:** A call sign is used for voice communication, but for digital calling via DSC, the MMSI is the fundamental "address." * **C) The vessel’s IMO number:** Similar to IMN, the IMO number is not the digital "address" used for radio communication. * **D) Vessel’s name and MMSI number:** This combination provides the essential digital contact information (MMSI) and a human-friendly label (name) for effective communication and identification within a marine radio address book.