Answer:
D: Former principle
Step-by-step explanation:
The prefix “ex” is used when describimg something “not” or “without” a title, status, etc. Its commonly used as ex-wife, ex-friend, indicating you’re not in a relationship with them anymore. In terms of the question it means the principle no longer works there.
Side Note: I hope this helps, feel free to let me know of you have any other questions :)