Explanation: VHF marine radio operates under international and national regulations (such as those from the ITU and FCC) which standardize channel assignments to ensure interoperability and minimize interference. For standard analog FM voice communications, the established channel spacing for VHF marine radio is 25 kHz. This 25 kHz spacing provides sufficient bandwidth for the transmitted FM voice signal and includes necessary guard bands to prevent interference with adjacent channels. While 12.5 kHz spacing is common for "narrowband" land mobile radio to conserve spectrum, the primary channel allocation and stepping for marine VHF systems historically and predominantly utilize 25 kHz. Smaller spacings like 10 kHz or 12.5 kHz would typically be too narrow for the full deviation of standard marine FM voice, and 20 kHz is not a recognized international standard for this service.