Answer: "The police officer was very concerned about what his daughter had done at school".
Explanation: A sentence fragment is a sentence that is incomplete because there is has an essential part missing. In "The police officer was very concerned about", the object of the preposition ("about") is missing. In that way, a subordinate clause was added to this sentence in order to make it a complex sentence. "What his daughter had done at school" is a subordinate clause because it does not express a complete thought; therefore, it cannot stand alone as a sentence. Moreover, "what his daughter had done at school", which has been introduced by the relative pronoun "what", is a noun clause since it is acting as a noun.
