When the IF signals swing above and below a center frequency, which system produces an audio output that fluctuates around zero?

The correct answer is the discriminator, as this system specifically converts frequency variations in the input signal into amplitude variations in the output. When the intermediate frequency (IF) signals oscillate around a center frequency, the discriminator processes these variations, leading to an audio output that reflects the fluctuations around a zero level.

In simpler terms, the discriminator takes the frequency-modulated (FM) signals, where the information is encoded in the frequency changes, and translates those changes into an audio signal that varies in amplitude. This allows for the extraction of the original message, and the resulting audio output can fluctuate around zero as it accurately represents the changes in frequency of the incoming signal.

Other systems, such as filters and demodulators, may play roles in signal processing but do not inherently produce an audio output that varies around a zero baseline in the same way that a discriminator does. Filters are primarily used to eliminate unwanted frequencies, and demodulators convert the modulated signal back into its original baseband signal, but they do not specifically indicate the fluctuation characteristic described in the question. Oscillators generate continuous waveforms and are not concerned with processing fluctuations of IF signals for audio output.