Explanation: In a fixed-frequency switching power supply, output voltage regulation is primarily achieved through Pulse Width Modulation (PWM). The switching circuit's "on" time, or pulse width, is adjusted to control the amount of energy transferred to the output. A) When the load impedance decreases, the load demands more current to maintain the regulated output voltage. To supply this increased current and maintain voltage, the power supply must transfer more energy from the input to the output during each switching cycle. This is accomplished by increasing the duration the switching element is "on," thus increasing the pulse width. B) If the load current decreases, less energy is needed to maintain the output voltage, so the pulse width would decrease. C) If the output voltage *increases* above its regulated set point, the feedback loop would reduce the pulse width to lower the voltage back to the desired level. D) If the input voltage increases, less "on" time (a smaller pulse width) is required to deliver the same amount of energy and maintain the regulated output voltage.