What is significant about using frequencies above 30 MHz?

Frequencies above 30 MHz are significant because they are less refracted by the atmosphere. This characteristic means that signals operating in this frequency range are more likely to travel in a straight line rather than bending around the curvature of the Earth or being influenced by atmospheric conditions. As a result, high frequencies do not benefit as much from ground wave propagation, which is the way signals can travel by hugging the ground. Instead, these higher frequencies are typically used for line-of-sight communication methods, which allows for clearer and more reliable transmission.

In addition, while some frequencies above 30 MHz are suitable for satellite communication, the critical aspect being highlighted in the question concerns their atmospheric propagation characteristics. The other choices do point to applications or properties related to different frequency ranges, but the defining factor for frequencies above 30 MHz remains their reduced refraction related to atmospheric influence.