Explanation: The critical information is that the terminal *does* log in successfully when connected to a *new* Above Deck Equipment (ADE) with a test cable. This demonstrates two things: 1. The Inmarsat satellite signal is available. 2. The terminal unit itself (below deck) is functional. Since the original ADE did not allow the terminal to log in, but a new ADE does, the problem is isolated to the original ADE or the original feedline. However, the direct evidence of a *working* new ADE strongly points to the original ADE being faulty. Replacing the faulty component, the ADE (D), is the logical next step. Options A and B are incorrect. Inmarsat-C terminals typically use omnidirectional or electronically steered antennas and do not have mechanical AZ/EL motors. Trying a different satellite (B) will not resolve a hardware fault in the ADE. Option C, while checking the feedline can be part of troubleshooting, a DVM is not ideal for assessing the RF performance (loss, impedance, noise) of a satellite feedline; specialized RF test equipment would be needed. More importantly, the test already confirmed the ADE is the likely culprit.