What are the dominant effect and frequency range for a space wave?

The correct choice highlights that the dominant effect associated with space waves is attenuation, particularly at frequencies above 30 MHz.

Space waves, which are typically associated with line-of-sight communication, rely on radio frequencies that can propagate through the atmosphere and travel directly to the receiver. At higher frequencies (greater than 30 MHz), space waves are less influenced by the ground or other obstacles, but they can still experience significant loss of signal strength, or attenuation, as they travel over distances. This is due to factors such as the absorption of the wave by the atmosphere and other environmental elements, which is particularly pronounced at higher frequencies.

In contrast, other answers might mention effects like diffraction or refraction, which are more relevant to different wave behaviors at lower frequencies or in different scenarios. For example, diffraction occurs at lower frequencies and under situations where waves need to bend around obstacles. Therefore, the relationship established in the correct answer between attenuation and frequencies above 30 MHz is logical and aligns with the principles of electronic warfare and radio wave propagation.