Explanation: Non-Directional Beacons (NDBs) transmit in the Medium Frequency (MF) band. During daylight hours, the D-layer of the ionosphere absorbs sky wave signals in this frequency range, meaning the aircraft's Automatic Direction Finder (ADF) primarily receives stable ground waves. At night, the D-layer dissipates. This allows MF signals to reflect off higher ionospheric layers (like the E and F layers) and return to Earth as sky waves. These reflected sky waves can interfere constructively or destructively with the direct ground wave signal arriving at the aircraft. This interference causes fluctuations in signal strength and erratic, unreliable bearing indications on the ADF, making navigation challenging. This phenomenon is known as the "night effect." Option A is incorrect because NDBs are critical navigation aids and operate continuously. Option C is incorrect; radio wave speed is constant in a given medium regardless of time of day, and ADF receives NDB signals. Option D is incorrect as "night effect" is a radio wave propagation phenomenon, not related to antenna moisture.