Explanation: A Voltage Controlled Oscillator (VCO) is a critical component of a frequency synthesizer in a VHF transmitter, generating the actual RF carrier frequency. Its frequency is precisely controlled by a Phase-Locked Loop (PLL) circuit, which compares the VCO's output to a stable reference and adjusts a control voltage to keep it locked to the desired frequency. An **out-of-lock condition in the Phased Locked-Loop circuit (B)** directly indicates that the PLL can no longer stabilize the VCO. This could happen if the VCO fails to oscillate, drifts excessively, or its control input circuitry malfunctions, preventing the PLL from maintaining phase lock. This is a primary diagnostic symptom for VCO failure. **A) Low transmitter power output in the high power mode** is usually indicative of issues in the power amplifier stage, antenna, or impedance matching, not directly the VCO. While a completely failed VCO would lead to no power, "low power" isn't the most specific symptom. **C) Excessive deviation in the frequency modulated output signal** relates to the modulator or audio processing stages, not the stability of the carrier frequency generated by the VCO. **D) A constant transmitter power output at various modulation levels** suggests problems with the modulation input or the power amplifier's linearity, not the VCO itself.