Frequencies in the UHF range normally propagate by means of:

The correct option is Space waves.

To understand why frequencies in the Ultra High Frequency (UHF) range propagate by means of space waves, we can analyze the characteristics of different radio wave propagation modes:
1. Ground waves (or Surface waves): These waves travel along the surface of the Earth. However, they suffer significant attenuation at higher frequencies because the ground absorbs energy from the wave. Ground wave propagation is effective only for low and medium frequencies (typically below 2 MHz). Therefore, UHF waves cannot propagate effectively as ground waves.
2. Sky waves: These waves travel upward and are reflected back to Earth by the ionosphere. The ionosphere can only reflect waves up to a certain critical frequency (usually below 30 MHz). Frequencies in the UHF range (300 MHz to 3 GHz) are too high to be reflected by the ionospheric layers; instead, they penetrate right through the ionosphere into outer space. Thus, sky wave propagation is not possible for UHF signals.
3. Space waves: Space waves travel in a straight line from the transmitting antenna to the receiving antenna. This mode of propagation includes direct line-of-sight propagation as well as waves reflected from the ground. Since UHF waves are not reflected by the ionosphere and are highly attenuated by the ground, they rely on this direct line-of-sight path (space wave propagation) to travel from the transmitter to the receiver.

Consequently, frequencies in the UHF range normally propagate by means of space waves.