Interference which is not unlawful? Do you mean lawful interception?
The Mode A/Mode C confusion comes from light aircraft equipment which has a rotary selector that goes OFF/STBY/ON/ALT where ALT in this case means Mode A with altitude reporting but can convey the impression that Mode C exists as a discreet selectable mode. This can also make (badly written) questions confusing because one does not know whether the chap that made the question up is asking you to choose 'Mode A with altitude reporting' or trying to trick you into choosing the wrong mode. Mode C is not available on its own and you cannot select a code, it sends the encoded pressure altitude from either the ADC or the digitiser on the instrument itself. There used to be a Mode B on some equipment as well, and of course there is mode S, a selection usually referred to on the rotary switch just as XPNDR.
With the current EASA exams it is always better to go back to the source regulation and you see from my previous post how the requirement is laid out in PANS-OPS, albeit for 7500. It is quite specific "Mode A Code 7500". Yes, altitude reporting would be nice to have and help everybody concerned but that is not the requirement.
I'm afraid I don't know who wrote your book, but they are not helping you.