Explanation: A common design goal for switching power supplies, including those in display systems, is to operate at a frequency above the range of human hearing. This prevents the power supply from producing an annoying audible whine or hum. The typical upper limit of human hearing is around 15-20 kHz. 18 kHz is a common switching frequency chosen precisely because it falls just above this range, making the power supply's operation inaudible. * **B) 120 Hz** is the ripple frequency from rectified 60 Hz AC mains, not a switching frequency for a modern supply. A supply switching at this frequency would be very noisy, large, and inefficient. * **C) 60 kHz and D) 120 kHz** are also common and often more efficient switching frequencies used in modern designs, allowing for even smaller components. However, 18 kHz represents a foundational and still common choice specifically for clearing the audible spectrum, making it a valid answer for "a common switching frequency." The choice of switching frequency also affects the characteristics of any Radio Frequency Interference (RFI) the supply might generate.