Which of the following best describes a shipboard Inmarsat-C system?

The description of a shipboard Inmarsat-C system as a "small, lightweight terminal capable of providing satellite store-and-forward message communications" accurately captures its primary function and characteristics. Inmarsat-C systems are designed specifically for sending and receiving text-based messages, including safety communications and distress alerts, without requiring large, complex equipment. The store-and-forward capability allows messages to be queued and transmitted even when the vessel is not in direct view of the satellite, which is particularly useful for flexibility in communication.

This highlights the system's utility in maritime environments, where real-time communication may not always be possible, and robust, reliable message handling is essential. The emphasis on being small and lightweight further underscores the practicality of Inmarsat-C for vessels operating in diverse conditions.

In contrast, the other options do not fully encompass the unique features of the Inmarsat-C system: it does not provide real-time connectivity like some other systems might, nor does it operate over high-frequency bands or guarantee continuous Digital Selective Calling coverage in the same way. This distinction makes option A the most accurate description of the Inmarsat-C system on board ships.