Which of the following effects may allow radio signals to travel beyond obstructions between the transmitting and receiving stations?

Knife-edge diffraction is a wave propagation phenomenon that allows radio signals to "bend" around sharp, solid obstacles like mountain ridges or the edges of large buildings. When a radio wave strikes the peak of such an obstruction, the edge acts as a secondary transmitting source, radiating the signal down into the shadow zone behind it. This effect is highly beneficial for VHF and UHF communications in mountainous or urban terrain, allowing stations to communicate even when there is no direct line of sight between them. Other options are unrelated to bending around physical objects: Doppler shift relates to frequency changes from relative motion, and Faraday rotation involves the changing polarization of a signal as it passes through the ionosphere.