Fill in the blank with the correct preposition.
He had to regret __________ what he had done.

for

The correct preposition to fill in the blank is for. The preposition for is used to indicate the purpose or reason for something. In this case, he had to regret for what he had done, which means that he felt sorry or remorseful for his actions.

The other options are incorrect.

• Of is used to indicate possession or relationship.
• Over is used to indicate something that is being discussed or talked about.
• At is used to indicate a specific point or place.