A vessel, before transiting the Panama Canal, on a voyage from San Diego to Miami, loses the ability to communicate via Inmarsat. The most likely cause is:

The correct answer is that the vessel has sailed beyond the coverage area of the Pacific satellite. Inmarsat communication relies on satellites placed in geostationary orbit, which provide coverage over specific areas of the Earth. As the vessel transitions from the Pacific Ocean into the Panama Canal, it is possible that the satellite previously providing communication has moved out of range due to the vessel's change in geographical location.

Inmarsat's coverage is divided into regions, and when moving from one region to another (such as from the Pacific to the Atlantic), the vessel may lose connection with the satellite that was servicing the area it just departed. This aligns with the scenario given, where loss of communication is most likely due to the vessel being beyond the coverage area of the Pacific satellite, causing it to no longer be able to establish a link for communication while sailing towards Miami.

The other options involve coverage from different stations or satellites that wouldn't be applicable during the vessel's passage at that specific point in the canal. Thus, the key aspect here is understanding the importance of regional satellite coverage associated with Inmarsat systems as vessels move through different geographic locations.