Explanation: An unlocked Phase Locked Loop (PLL) circuit in the frequency synthesizer is the direct cause of a receiver failing to stay tuned. The PLL generates the stable local oscillator frequency required for mixing with the incoming signal. If it unlocks, this crucial frequency drifts or becomes unstable, preventing the receiver from accurately locking onto and staying tuned to the desired channel. A) The local oscillator beating against the incoming signal to produce an Intermediate Frequency (IF) describes the normal, essential operation of a superheterodyne receiver, not a fault preventing tuning. C) A discriminator circuit developing positive and negative voltages at the modulation frequency is the normal function of an FM detector, recovering the audio information. D) A ratio detector producing a variable output within the receiver’s dynamic range describes its proper operation in demodulating an FM signal, not a tuning instability.