The correct option is D take over
The context refers to someone resigning and someone else taking their place. In this context, option A "take over" means to take control. Thus, this is correct.
To "take in" is to give shelter to someone.
To "take up" is to start a new hobby or to begin something.
To "take off" is to become airborne or to depart hastily.
Thus, option A is correct.