Explanation: A properly adjusted limiter circuit functions by automatically reducing the amplitude of signal peaks that exceed a predetermined threshold. Its purpose is to prevent the output signal's amplitude from rising above this set level, thereby maintaining a relatively constant output amplitude even when the input signal's amplitude varies significantly. This is particularly important in FM transmitters, where a limiter (often part of a speech processor) prevents over-deviation of the carrier, ensuring a clean signal and consistent modulation levels without distortion. * **A) Maintains a constant output amplitude with a varying input amplitude.** This is correct. The limiter compresses or clips the signal peaks, ensuring the output stays within a desired amplitude range. * **B) Limits the level of RF output power in the final amplifier stage.** While a limiter can *indirectly* affect average power by allowing higher modulation without over-deviation, its direct function isn't to limit the *RF output power* of the final amplifier. That's typically done by power control circuits. * **C) Limits the modulation input amplitude while amplifying the modulation output.** A limiter *does* limit the modulation input amplitude, but its primary function is not to *amplify* the modulation output. It's about amplitude control, not gain. * **D) Controls the DC input voltage excursions of the final amplifier stage.** This describes the function of a power supply regulator, not an amplitude limiter, which operates on the AC signal component.