The correct answer is D: Temperature inversions in the atmosphere. Tropospheric ducting is caused by temperature inversions in the atmosphere - situations where the air temperature increases with altitude instead of decreasing (which is normal). These inversions create a 'duct' that can trap and guide radio signals. Normally, air temperature decreases with altitude. But during temperature inversions (often associated with certain weather patterns), a layer of warm air sits above cooler air. This creates a boundary that can act like a waveguide, trapping VHF and UHF signals and allowing them to travel much farther than normal. Temperature inversions are common in certain weather conditions and geographic areas, which is why tropospheric ducting is a regular occurrence in some regions.