Why A phase-locked loop is a feedback (servo) control system for frequency. A phase detector compares the output of a voltage-controlled oscillator (usually after a divider) against a stable reference oscillator, the resulting error voltage is smoothed by a low-pass loop filter, and that filtered voltage steers the VCO until its phase and frequency track the reference. Those four blocks, phase detector, low-pass filter, VCO, and reference oscillator, are the defining parts, and the arrangement is the basis of frequency synthesizers and FM demodulators.
Watch out The choice listing a ratio detector and reactance modulator names FM discriminator and modulator parts, not the PLL blocks; a monostable multivibrator is a one-shot timer with no frequency-locking feedback at all.
PLL = Phase detector, Low-pass filter, VCO, plus reference. Look for the answer with all four.