The correct option is C get off
The context refers to someone being guilty. This implies a crime of some sort. So, option C "get off" which means to receive no or almost no punishment is correct. In the context it indicates "he won't get off" which means he will receive punishment.
"Get about" in option A means moving from place to place.
"Get across" in option B indicates succeeding in communicating something.
"Get away" in option D means running away or escaping from a place of danger. This does not concur with the context which implies punishment for some crime of which the person is guilty.
Thus, option C is correct.