What type of propagation is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?

Answer: A. Tropospheric ducting

Tropospheric ducting traps VHF and UHF signals in a layer of the lower atmosphere, carrying them over the horizon to around 300 miles on a regular basis. F2 region refraction is an HF effect.

Tip: Ducting: signals travel along an atmospheric "pipe."