Final answer:
The dependent clause in the given sentence is 'that she found on the floor,' which cannot stand alone as a complete sentence and contains a subject and a verb.
Step-by-step explanation:
The dependent clause in the sentence 'The police officers thanked Haley Jackson for returning a wallet that she found on the floor.' is the phrase 'that she found on the floor.' This clause cannot stand alone as a complete sentence because it does not express a complete thought without the independent clause it attaches to. The clause 'that she found on the floor' begins with 'that', which is a subordinating conjunction, and contains both a subject ('she') and a verb ('found'), thus fulfilling the criteria of a dependent clause.