Which system has the least effective radius of operation?

VHF DSC (Digital Selective Calling) systems operate primarily in the Very High Frequency band, which is effective for line-of-sight communication. The range of VHF communications is generally limited to approximately 30 nautical miles, depending on factors such as the height of the transmitting and receiving antennas and the curvature of the Earth. This relatively short range makes VHF DSC less suitable for long-distance communication compared to HF and MF systems.

In contrast, HF SITOR (Single Sideband Telex Over Radio) and MF SITOR utilize lower frequency bands that can reflect off the ionosphere, allowing for greater distances of operation, potentially thousands of miles, particularly in favorable atmospheric conditions. NAVTEX, a broadcast service for maritime navigational and meteorological information, also has a broader range compared to VHF DSC, as it operates in the MF band. Hence, within the context of these choices, VHF DSC indeed has the least effective radius of operation, making it the correct selection.