Explanation: Crystal oscillators are precise frequency sources, and their stability is crucial for clear and consistent transmission. The output frequency of a crystal oscillator can be sensitive to fluctuations in its supply voltage. When the oscillator shares a power supply with other stages of a transmitter, particularly power amplifiers which draw large, varying currents, these current changes can cause voltage drops and ripple on the shared supply line. This unwanted variation in the oscillator's supply voltage can lead to "dynamic instability," where the oscillator's frequency drifts or "chirps" during operation, especially when modulated or under varying load conditions from subsequent amplifier stages. Providing a separate, well-regulated power source isolates the crystal oscillator from these load-induced voltage fluctuations, ensuring a stable operating voltage and thereby preventing dynamic instability and maintaining a constant output frequency. Options B (thermocoupling) and C (overtuning) describe unrelated phenomena or issues. Option D is incorrect because the goal is always to achieve stability, not instability, in a frequency source.