From the given options, fill in the blank with the most suitable preposition :

The queen was concerned _________ the safety of the princess.

about

The answer is about.

The preposition about is used to express concern for someone or something. In this case, the queen is concerned about the safety of the princess.

The other prepositions are not as suitable. At is used to indicate a specific place or time. With is used to indicate accompaniment or association. Of is used to indicate possession or relationship.