Explanation: Switching power supplies operate at high frequencies (typically tens of kHz to MHz). At these frequencies, transformers constructed with laminated steel cores, common for 50/60 Hz mains, would experience significant losses due to eddy currents and hysteresis. **A) Transformers use a ferrite core to reduce losses at high frequencies.** This statement is correct. Ferrite materials have high magnetic permeability and high electrical resistivity, making them ideal for high-frequency transformer cores. They effectively minimize eddy current losses and hysteresis losses, which are critical for the efficiency of switching power supplies. **B) Transformers use a laminated steel core to reduce losses at high frequencies.** This is incorrect. Laminated steel cores are used to reduce eddy current losses at *low* frequencies (like 50/60 Hz). At high frequencies, even laminated steel cores would have unacceptably high losses. **C) Lower switching frequencies provide better voltage regulation.** This is incorrect. Higher switching frequencies generally allow for smaller filter components and faster response times to load changes, often leading to *better* or more efficient voltage regulation. **D) Higher switching frequencies are more difficult to filter.** This is incorrect. While higher frequencies can introduce challenges with electromagnetic interference (EMI) radiation, filtering out the *ripple* from the switching frequency itself is generally *easier* with smaller, more effective filter components (capacitors and inductors) as frequency increases.