Review the passage:
Phillip left work early. To memorize the vocabulary that his Spanish midterm
would test the next day. But all that he could think
about was Beatrice, the cute new sales associate who worked in the electronics
department.
You chose the third part, But all that he could think about was Beatrice, the cute new sales associate who worked in the electronics department. This part has a main clause: All is the subject; was is the verb. A fragment will not have a main clause.