Which of the following would reduce RF interference caused by common-mode current on an audio cable?
Why Common-mode current is RF flowing in the same direction on every conductor of the cable, including the shield, so the cable acts like an antenna feeding RF into the audio gear. A ferrite core clamped around the whole cable adds series impedance only to that common-mode current, because the desired audio signal travels as equal and opposite currents whose fields cancel inside the core. Several turns of the cable through the ferrite increase the choking impedance further.
Watch out Shorting center conductor to shield would simply kill the audio signal, and grounding the center conductor does the same; neither addresses the RF riding on the outside of the cable. Extra insulation does nothing, since common-mode current is conducted on the wire, not leaking through the jacket.
Ferrite chokes the common mode, passes the differential signal. Think of it as a one-way speed bump for RF on the outside.