Explanation: INMARSAT-B, like all satellite communication systems, relies on a clear line of sight between the user terminal (Ship Earth Station - SES) and the satellite. * **A) Obstruction:** Any physical blockage, such as a ship's mast, bridge, or even landmass, between the SES antenna and the satellite will attenuate or completely block the radio signal, preventing communication. This is a fundamental aspect of radio propagation, especially at the higher frequencies used by satellites, which behave more like light rays. * **B) Low elevation/below the horizon:** If the satellite's apparent position is too low in the sky, or even below the horizon from the SES, the Earth itself (or its curvature) obstructs the signal path. For reliable communication, the satellite must be above a minimum elevation angle, typically around 5-10 degrees, to avoid excessive atmospheric interference and ground clutter. * **C) Travel beyond the effective radius:** Geostationary satellites, like those used by INMARSAT, cover a specific geographical "footprint." Traveling outside this designated service area means the satellite's transmitted signal is too weak to be reliably received, or the user is outside the designed coverage pattern, making communication impossible. Since all three conditions represent distinct and valid reasons for loss of satellite communication, "All of these" is the correct answer.