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

Radio waves normally travel in a straight line, but they can bend or diffract around sharp edges, such as the peak of a mountain or the corner of a tall building. This phenomenon is known as knife-edge diffraction. Because the radio wave strikes the sharp edge, it scatters and bends slightly downward into the shadow region behind the obstruction. This allows communication to occur even when there is no direct line of sight between the transmitter and receiver. The other options do not fit this scenario. Faraday rotation deals with the polarization of waves in the ionosphere, Doppler shift is the change in frequency due to relative motion, and quantum tunneling is a microscopic physics concept, making them incorrect for this macro-scale propagation effect.