Regarding (C), Human souls are not really “defined” to be immortal; (B) would be closer to “God created human souls which are immortal.” Human intellectual operations are immaterial, so the human soul is immaterial. It follows not that it is a logical contradiction for a soul to go out of existence, but that the human soul (being immaterial) cannot be destroyed by material means. That says nothing of whether God could destroy a human soul.
God would not even “need” to destroy a human soul. Catholics believe that God sustains everything (including that which is immaterial) in existence at every moment. For a soul to go out of existence, God would just have to stop sustaining it. This is all counterfactual, though, since God, having willed a human to exist, would not change His will (since He is immutable) and stop sustaining the soul.
So it would be “contradictory” for a human soul to go out of existence, but it is not a logical contradiction derived from the definition of “immortal,” since when we say that a soul is immortal, we are not quite saying that it is exists necessarily, just that it cannot be destroyed materially.