Which system has the least effective radius of operation?

Correct Answer: Option C — VHF DSC

VHF radio propagation is strictly limited to line-of-sight, typically 20 to 30 nautical miles depending on antenna height. Unlike MF/HF NBDP or NAVTEX, which utilize ground-wave or sky-wave propagation to achieve ranges of hundreds or thousands of miles, VHF DSC is inherently restricted to short-range, local maritime communications.