Explanation: Compulsory radiotelephone stations are required by international and domestic regulations (such as FCC Part 80) to maintain a continuous listening watch on **VHF channel 16 (156.8 MHz)** when underway. Channel 16 is the international distress, safety, and calling frequency for maritime voice communications. This ensures that vessels can receive distress calls, urgent safety information, and establish initial contact with other stations. Option A, VHF channel 22A, is a U.S. Coast Guard working channel, not the primary international distress and calling watch channel. Option B, 121.5 MHz, is an aeronautical emergency frequency and was historically used by emergency locator transmitters (ELTs/EPIRBs), but it is not a maritime radiotelephone watch frequency for vessels. Option D, 500 kHz, was the international distress frequency for Morse code telegraphy but has been phased out and replaced by modern GMDSS (Global Maritime Distress and Safety System) components, including VHF Channel 16.