Explanation: The observation of 25 watts output indicates the radio's RF power amplifier is functioning and transmitting a carrier wave. However, the lack of modulation, despite the carrier, means no audio information is being superimposed onto that carrier. An open connection in the microphone cable or connector (B) prevents the audio signal from the microphone from reaching the radio's modulator circuit. The transmitter keys up and generates an RF carrier, but with no audio input, the carrier remains unmodulated, resulting in silence on the receiving end. Options A, C, and D are less likely to cause a complete absence of modulation while still producing full RF output: * (A) A PTT fault might prevent transmission altogether or cause other issues, but not typically a silent carrier with full power. * (C) An AGC malfunction would likely result in audio that is too loud, too quiet, or distorted, not completely absent. * (D) Excessive degenerative feedback in the AF amplifier would significantly reduce audio gain, but usually wouldn't eliminate all audio input, especially if the microphone itself is working.