(A) is an description of Avalanche breakdown, not Zener breakdown.
(B) is incorrect, this is due to tunneling of electrons not protons.
(C) is correct - Under a high reverse-bias voltage, the p-n junction's depletion region expands, leading to a high strength electric field across the junction. A sufficiently strong electric field enables tunneling of electrons from the valence to the conduction band of a semiconductor leading to a large number of free charge carriers. This sudden generation of carriers rapidly increases the reverse current and gives rise to the high slope conductance of the Zener diode.
(D) is also incorrect, it has nothing to do with decrease in depletion layer.