Explanation: The Global Maritime Distress and Safety System (GMDSS) is fundamentally designed as an international framework for maritime distress, urgency, and safety communications. Its primary basis is to ensure that a distress alert is received ashore by search and rescue (SAR) authorities and by shipping in the immediate vicinity of a vessel in distress, facilitating a coordinated and effective response. Option D correctly identifies this core principle: linking SAR authorities ashore with nearby vessels to mount a rescue. GMDSS achieves this through a combination of satellite communications, digital selective calling (DSC) on VHF, MF, and HF, and other systems like NAVTEX. Options A, B, and C describe specific communication methods or components *within* the GMDSS (ship-to-ship radiotelephony, VHF DSC, and narrow-band direct printing, respectively). While these are vital parts of the system, they are individual tools that contribute to the overall coordinated response, rather than representing the primary, overarching basis of the GMDSS itself. The system's strength lies in its comprehensive, integrated approach.