Explanation: Crystal oscillators require positive feedback to initiate and sustain oscillation. The crystal provides the frequency-determining element, but a portion of the output signal must be fed back to the input, with the correct phase, to overcome circuit losses and maintain continuous oscillation. An additional plate-grid feedback capacitor (or equivalent in solid-state circuits, such as collector-base or drain-gate) provides this crucial feedback path. It couples energy from the output back to the input, ensuring the necessary phase shift (often 180 degrees in conjunction with the active device's 180-degree shift) to create continuous, regenerative feedback and oscillation. * **B) Adding a coupling circuit in series with the plate resistors** is incorrect. A coupling circuit typically transfers signals between stages or to a load, not directly as the primary feedback mechanism to sustain oscillation within an oscillator. Plate resistors primarily set operating bias. * **C) An astable multivibrator** is incorrect. An astable multivibrator is a free-running oscillator itself, generating square waves, but it does not use a crystal for frequency stability and is not a component added *to* a crystal oscillator to provide feedback. * **D) A monostable multivibrator** is incorrect. A monostable multivibrator is a "one-shot" circuit that generates a single pulse when triggered; it is not an oscillator and has no role in providing feedback for a crystal oscillator.