Explanation: VHF 121.5 MHz is the international aeronautical emergency frequency. Aircraft, especially those involved in search and rescue (SAR) operations or responding to distress calls, are equipped to monitor and communicate on this frequency. Therefore, for "on scene" coordination between a vessel and aircraft during an emergency or SAR event, 121.5 MHz is the designated and most effective channel. Conversely, 9 GHz is in the microwave band and primarily used for radar, not voice communications. VHF Channel 13 is designated for bridge-to-bridge navigation safety communications between vessels, not for communication with aircraft. VHF Channel 26 is a public correspondence (ship-to-shore telephone) channel and is not monitored by aircraft for emergencies.