Explanation: The Global Maritime Distress and Safety System (GMDSS) fundamentally aims to ensure a distress alert from a vessel reaches appropriate rescue authorities ashore, and/or vessels in the immediate vicinity, as quickly and reliably as possible. **Answer D is correct** because it accurately describes this core principle. GMDSS integrates satellite and terrestrial radio communication systems (VHF, MF, HF DSC, Inmarsat) to achieve this global coordination, linking those in distress with those who can provide assistance, whether they are on land (Search and Rescue centers) or at sea (other ships). **Answer A is incorrect** because while ship-to-ship distress communication is a component of GMDSS, it is not the *primary basis* for the entire system, which also heavily involves shore-based SAR coordination and satellite communications. **Answer B is incorrect** as VHF Digital Selective Calling (DSC) is a vital *method* within GMDSS for alerting in coastal waters, but it is only one part of the broader, global system that includes MF, HF, and satellite communications for wider coverage. **Answer C is incorrect** because Narrow-Band Direct Printing (NBDP) is a communication mode used within GMDSS (e.g., NAVTEX, SITOR), but it's not the *primary basis* for all distress, urgency, and safety communications. DSC and voice radiotelephony are also critical.