Which system has the least effective radius of operation?

Correct Answer: Option D — VHF DSC

VHF DSC operates via line-of-sight propagation, heavily restricted by the Earth's curvature and antenna height, giving it the shortest effective radius (typically 20 to 30 miles). Conversely, NAVTEX, HF SITOR, and MF NBDP utilize groundwave and skywave propagation, providing much broader regional or long-range operational coverage.