Which of the following systems is least susceptible to atmospheric disturbances?

Inmarsat is a satellite communication system that operates through a network of geostationary satellites. Because it relies on satellite technology, it is generally less affected by atmospheric disturbances compared to systems that operate on terrestrial radio frequencies.

Satellite communications, such as those provided by Inmarsat, have the advantage of functioning above the Earth's atmosphere, which allows them to maintain stable signals even in adverse weather conditions like storms or heavy precipitation. This is in contrast to systems like MF SITOR and HF SITOR, which operate on medium and high-frequency bands respectively, both of which can be significantly impacted by atmospheric factors such as ionospheric changes, fading, and interference caused by weather conditions.

NAVTEX, while being a reliable system for broadcasting navigational and meteorological warnings, also relies on terrestrial-frequency transmission which makes it more susceptible to atmospheric disturbances compared to satellite-based systems. Therefore, Inmarsat's reliance on satellite technology makes it the least susceptible choice among the systems listed.