The correct answer is D: Through a quartz crystal. In a Pierce oscillator, positive feedback is supplied through a quartz crystal. The Pierce oscillator is a crystal-controlled oscillator that uses the crystal as both the frequency-determining element and the feedback element. The crystal provides the precise frequency control and the phase shift necessary for oscillation. The Pierce oscillator is one of the most common crystal oscillator circuits because it's simple, stable, and reliable. The crystal acts as a high-Q resonant circuit that determines the exact oscillation frequency. This makes Pierce oscillators ideal for applications requiring precise frequency control, such as in transceivers, frequency standards, and digital clock circuits.