With frequencies in the _____________ range, either horizontal or vertical polarization is satisfactory.

The correct choice indicates that at very high frequencies, either horizontal or vertical polarization is satisfactory for radio wave propagation. This phenomenon occurs due to the properties of radio waves as they interact with the environment at these frequencies.

At very high frequencies, such as those used for microwave communications (typically above 300 MHz), the effects of polarization become less critical than at lower frequencies. This is because the shorter wavelengths associated with very high frequencies are less susceptible to the depolarizing effects of terrain features, buildings, or other obstructions. Thus, both horizontal and vertical polarization can achieve comparable effectiveness in terms of signal strength, quality, and reliability.

In contrast, lower frequency ranges might show a preference for one polarization due to factors such as ground wave propagation and interaction with obstacles, where one form of polarization may perform better than the other. For this reason, knowing the implications of polarization at different frequency ranges is crucial for optimizing communication systems.