Fill in the blank with the correct preposition.
Mother was anxious __________the safety of her daughter.

about

The correct preposition to fill in the blank is about. The preposition about is used to indicate concern or worry. In this case, mother was anxious about the safety of her daughter, which means that she was worried about her daughter's safety.

The other options are incorrect.

• Upon is used to indicate something that is being discussed or talked about.
• For is used to indicate purpose or destination.
• At is used to indicate a specific point or place.