Which of the following components can be used to reduce thermal drift in crystal oscillators?
Why A crystal oscillator's frequency depends on the crystal plus the loading capacitance around it, so any capacitor whose value drifts with temperature drags the frequency with it. NP0 (also called C0G) ceramic capacitors are specified for a near-zero temperature coefficient, roughly 0 +/- 30 ppm per degree C, so the load capacitance stays put as the circuit warms up. Using them in the frequency-determining part of the oscillator is the standard cure for thermal drift.
Watch out Toroidal inductors are chosen for self-shielding and high Q, not temperature stability, and resistor types (wirewound or non-inductive) affect inductance and noise, not the oscillator's frequency-setting reactance.
NP0 = Negative-Positive-Zero drift: zero temperature coefficient, zero frequency wander.