Explanation: NAVTEX receivers depend on a strong, clear signal at medium frequencies to decode incoming messages. An RF amplifier, often located near the antenna base, is critical for boosting weak signals and compensating for cable losses, ensuring sufficient signal strength reaches the main receiver unit. If this RF amplifier becomes defective, it will significantly degrade or entirely eliminate the incoming signal before it reaches the receiver's processing circuits. The NAVTEX unit itself may power on and appear normal (displaying old messages, time, etc.) because its internal components are functioning, but it will be unable to successfully receive and decode any new, weak radio signals. The other options are less likely: * **A) The audio amplifier is defective:** This would prevent audible alarms or message readouts, but the unit would still *receive* and display messages. * **B) The wrong frequency has been selected:** This is explicitly contradicted by the problem statement that "The station settings are correct." * **D) The DC supply voltage is too low:** While low voltage can cause various malfunctions, it's more likely to cause erratic behavior or prevent the unit from powering on at all, rather than just silently disabling reception while appearing otherwise normal.