Which frequencies are typically used for communications beyond continental airspace?

Answer: C — High Frequency (HF)

The use of High Frequency (HF) for communications beyond continental airspace is appropriate due to the unique properties of HF radio waves. HF frequencies, which range from 3 to 30 MHz, can reflect off the ionosphere, allowing for long-distance communication. This is especially crucial over vast oceanic areas and remote regions where line-of-sight communication via VHF is not feasible. HF is the primary choice for transoceanic flights, as it can provide coverage over large distances that exceed the range capabilities of VHF. While VHF is used for shorter-range communications, particularly in terminal airspace and below 24,000 feet, it is not reliable for long distances due to its line-of-sight limitations. Medium Frequency (MF) frequencies are typically used for maritime communications and navigational aids, and Ultra High Frequency (UHF) is generally used for military aviation and near the ground operations. Consequently, HF is essential for maintaining contact with aircraft flying over extensive areas of ocean where other communication methods are out of reach.