Explanation: The High Frequency (HF) band is internationally defined as the spectrum ranging from 3 to 30 MHz. This range is particularly important for amateur radio operators as it supports long-distance communication, primarily through ionospheric reflection, where radio waves bounce off layers in the Earth's upper atmosphere, allowing signals to travel over the horizon. Option A, 300-3,000 kHz (or 0.3-3 MHz), primarily defines the Medium Frequency (MF) band. Option B, 30-300 MHz, corresponds to the Very High Frequency (VHF) band. Option C, 10-30 MHz, covers only a portion of the HF spectrum, specifically the upper half, and does not represent the full range. Therefore, 3-30 MHz is the correct and complete definition for the High Frequency band.