Which of the following might be useful in reducing RF interference to audio frequency circuits?
Why A capacitor's reactance falls as frequency rises (Xc = 1/(2 pi f C)), so a small capacitor connected from a signal line to ground looks like a near short to RF while remaining a high impedance at audio frequencies. That shunts the unwanted RF energy to ground before it can be detected and rectified in the audio stage, and the wanted audio passes through untouched. Bypass capacitors at amplifier inputs, speaker leads, and power pins are a standard cure for RF getting into stereos, telephones, and intercoms.
Watch out An inductor used as a shunt would do the opposite, presenting low impedance at audio and high impedance at RF, which kills the wanted signal and leaves the interference; inductors help when placed in series with a lead, as in a ferrite choke.
Caps shunt highs to ground: bypass capacitor = RF drain, audio stays.