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:

Correct Answer: Option D — The vessel has sailed beyond the coverage area of the Pacific satellite.

The Pacific Ocean Region (POR) satellite coverage does not extend to the Caribbean or the Atlantic side of the Panama Canal. A vessel transiting from San Diego to Miami would eventually move out of the POR satellite's footprint, resulting in a loss of Inmarsat connectivity as it transitions to Atlantic satellites.