Explanation: A phase-locked loop (PLL) frequency synthesizer generates a stable output frequency that is a multiple of a very stable reference frequency. The components in option D form the core of such a system: * **Master oscillator (or reference oscillator):** Provides a highly stable, fixed frequency reference. This is the source of the PLL's ultimate frequency stability. * **Frequency dividers:** These divide the VCO's output frequency down and/or divide the reference frequency. By changing the division ratio, the output frequency of the synthesizer can be changed while maintaining lock with the stable reference. This is crucial for frequency *synthesis*. * **Phase comparator (or phase detector):** Compares the phase difference between the divided-down VCO signal and the reference signal. It generates an error voltage proportional to this phase difference. * **Loop filter:** Filters and smoothes the error voltage from the phase comparator, converting it into a stable DC control voltage for the VCO. It also determines the loop's bandwidth and stability characteristics. * **Voltage Controlled Oscillator (VCO):** Generates the output frequency, whose frequency is directly controlled by the DC voltage from the loop filter. These components work in a feedback loop: the VCO's output is divided and fed back to the phase comparator, which compares it to the reference. The resulting error voltage adjusts the VCO's frequency until it locks, meaning the divided-down VCO frequency matches the reference frequency (or a fixed ratio), thereby synthesizing a stable output frequency. Options A, B, and C are incorrect because they either include components not central to a basic PLL synthesizer (e.g., ratio detector, limiting amplifier, high-pass filter, or a *first mixer stage* in place of essential dividers for synthesis) or, more critically, they omit fundamental components like the frequency dividers (essential for synthesis) or the loop filter (essential for stable operation).