Explanation: A pulse-width modulator (PWM) in a switching power supply is fundamental to its regulation. Its primary function is to vary the **duty cycle** of the switching transistor. The duty cycle is the ratio of the time the switch is in the "ON" state to the total switching period. By adjusting this ratio, the PWM controls the amount of energy transferred from the input to the output of the power supply during each switching cycle. A higher duty cycle means the switch is on for longer, transferring more energy and typically resulting in a higher output voltage (or current), while a lower duty cycle reduces the energy transfer and lowers the output. This precise control allows the power supply to maintain a stable, regulated output voltage despite changes in input voltage or load. * **A) Provide the reference voltage for the regulator.** This is incorrect. The reference voltage is a stable, fixed voltage used as a set point for the regulator's feedback loop, which the output is compared against. The PWM uses the error signal derived from this comparison. * **B) Vary the frequency of the switching regulator to control the output voltage.** While some advanced designs use frequency modulation, the most common and fundamental method for regulation in switching power supplies is pulse-width modulation, where the switching frequency is typically constant, and the duty cycle is varied. * **D) Compare the reference voltage with the output voltage sample and produce an error voltage.** This describes the function of the error amplifier or comparator within the feedback loop, which *generates* the error signal that the PWM then uses to determine the appropriate duty cycle.