Not having seen a 172 recently I can only remember the stall warning system on the 150/152, this uses a slot in the leading edge of the wing that is connected to a horn in the cockpit, the reduction in air pressure at the leading edge due to the movement of the stagnation point as a result of increasing angle of attack activates the horn.
This system can be checked on the ground by placing your lips on the slot in the wing and sucking...
I believe I recall some of the very early C172s having electric stall warning systems, but the vast majority are exactly as A and C suggests.
MJ
