Why Tropospheric ducting happens when a temperature inversion creates a layer of warm, dry air over cooler, moist air, forming a waveguide-like duct that traps VHF/UHF/microwave signals and keeps them from spreading upward. Signals can travel far beyond the normal radio horizon with low loss, and the commonly cited working range is on the order of 100 to 300 miles. Exceptional ducts over water can stretch farther, but the pool's expected answer is the few-hundred-mile figure.
Watch out The 10 to 50 mile choice is just ordinary line-of-sight VHF/UHF range with no duct at all, and the 1,200 and 2,500 mile figures are more typical of ionospheric F-layer HF hops, which microwaves do not use.
Ducting = a few hundred miles: remember 100-300. Anything over 1,000 miles is ionospheric, not tropospheric.