A radio wave that travels along the surface of the earth is referred to as what?

The term that describes a radio wave traveling along the surface of the Earth is "ground wave." Ground waves propagate by following the contour of the Earth, effectively utilizing the surface as a medium for transmission. This type of wave is particularly beneficial for low-frequency radio communications, as these frequencies can diffract around obstacles and thus maintain signal strength over longer distances. Ground waves are affected by the conductivity of the ground and the frequency of the radio wave, making them effective for local broadcasting applications. This is why the choice indicating "ground wave" is the appropriate answer in this context.