Take a look at the passage again:
Head down, Lela stared at the textbook on her desk.
She understood the fragment practice that Mrs.
Markham was going over in class. But was too shy to raise her hand
and volunteer an answer.
You chose the second part, She understood the fragment practice that Mrs. Markham was going over in class. This part has a main clause: She is the subject; understood is the verb. A fragment will not have a main clause.
Go back to the passage to try again.