Which of the following factors can negatively impact radio wave propagation?

Obstruction by mountains or tall structures significantly impacts radio wave propagation because these physical barriers can block or attenuate the signals. When radio waves encounter a mountain or a tall building, a portion of the signal can be reflected, refracted, or absorbed, which can lead to a weaker reception or loss of signal altogether.

In contrast, high humidity levels may not necessarily have a negative impact on propagation as it can affect certain frequencies differently, sometimes even enhancing the propagation of lower frequencies. Increasing antenna height typically improves the range and effectiveness of transmission, allowing signals to travel further and with less obstruction. The use of high-powered transmitters is generally advantageous for sending radio signals over greater distances, as it increases the signal strength. Thus, the presence of large physical obstructions poses the most significant challenge to effective radio wave propagation.