Explanation: Modern satellite terminals, like the Inmarsat Fleet 77, incorporate sophisticated internal diagnostic capabilities. When a critical internal communication path, such as between the second receiver (RX2) and the Antenna Control Unit (ACU), fails, the terminal's operating system is designed to detect this anomaly. It then presents a clear, user-friendly error message on the display to alert the operator. Therefore, an explicit message like "RX2 Comm error" or "ACU Comm error" is the direct indication of such a communication breakdown. Options A and B describe successful states (locking onto a signal or receiving a valid frame), which are the opposite of a communication failure. Option D, concerning DCE/DTE communication protocol configuration, typically refers to external interface settings (e.g., connecting a computer to the terminal) and is not usually relevant to the internal, hard-coded communication between the RX2 and ACU modules within the terminal itself.