Explanation: An Inmarsat-C terminal requires converted DC power from its internal power supply to operate. If AC voltage is present at the power supply's input terminals but the terminal fails to power up, it indicates the power supply itself is not successfully converting the AC into the necessary DC voltages. A component failure within the power supply (e.g., rectifier, transformer, regulator) would prevent this conversion, thus preventing the terminal's internal circuits from receiving power and initiating the power-up sequence. A defective coaxial cable (A) or a failed I.F. amplifier stage (B) would prevent the terminal from transmitting or receiving signals, respectively, but not from powering on. A defective video monitor (D) would prevent displaying information but would not stop the terminal from powering up internally. These issues relate to operational functionality, not the fundamental ability to energize the system.