The correct answer is C: UHF signals are usually not propagated by the ionosphere. Simplex UHF signals are rarely heard beyond their radio horizon because UHF frequencies are generally too high for ionospheric propagation. The ionosphere typically doesn't refract UHF signals back to Earth. UHF signals (300-3000 MHz) are above the maximum usable frequency (MUF) for normal ionospheric propagation. While the ionosphere can sometimes propagate lower UHF frequencies under exceptional conditions, normal UHF communications are line-of-sight. UHF signals travel in straight lines and are blocked by the curvature of the Earth beyond the radio horizon. This is why UHF is excellent for local communications but doesn't provide the long-distance capabilities of HF.