(1) Ground Waves
(2) Sky Waves
(3) Surface Waves
(4) Space Waves
ANSWER – Space Waves
Explanation – An ultra-high frequency (UHF) wave cannot be reflected back by the ionosphere as the highly energetic signal waves penetrate the ionosphere and escape into the upper atmosphere. Also, these waves can not travel along the trajectory of the ground. Hence, propagation of ultrahigh-frequency signals is achieved through the line – of – sight communication, which is in fact space wave propagation.