By alternating the output between positive and negative voltages
By varying the duty cycle of pulses input to a filter✓Правильный ответ
By varying the conductivity of a pass element
By switching between two Zener diode reference voltages
Объяснение
A switchmode regulator chops the input into a train of pulses and feeds them to an inductor-capacitor filter that averages them. The average, and therefore the output voltage, is set by how wide the pulses are, so regulation is achieved by varying duty cycle rather than by dissipating the difference.