Take a look at the passage again:
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 camera department.
You chose the first part, Phillip left work early. This part has a main clause: Phillip is the subject; left is the verb. A fragment will not have a main clause.
Go back to the passage to try again.