An electronic servo loop consisting of a ratio detector, reactance modulator, and voltage-controlled oscillator
An electronic circuit also known as a monostable multivibrator
An electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator✓Correct answer
An electronic circuit consisting of a precision push-pull amplifier with a differential phase input
Explanation
A phase-locked loop is a servo loop: a phase detector compares a stable reference with the output of a voltage-controlled oscillator, and the resulting error, after smoothing, tunes the VCO until the two stay locked in phase.