Explanation: In an FM receiver, the Continuous Tone-Coded Squelch System (CTCSS) tone is a sub-audible tone that frequency modulates the carrier along with the voice. The discriminator stage is responsible for demodulating the FM signal, converting the frequency variations into corresponding voltage variations. At the output of the discriminator, both the voice signal and the sub-audible CTCSS tone are present as voltage signals in the baseband audio. To ensure the listener doesn't hear the CTCSS tone, it must be filtered out *after* it has been successfully extracted from the FM carrier by the discriminator, but *before* the signal reaches the audio amplifier and speaker. Option D correctly places the filtering process after the demodulation (discriminator) and before the final audio stages, where a low-pass filter removes the sub-audible tone from the audible path. Options A, B, and C are incorrect because at the IF or mixer stages, the CTCSS tone is still part of the frequency modulation and has not yet been converted into a distinct, filterable voltage signal. Filtering at these stages would prematurely distort or remove the desired signal components.