Review the passage:
Head down, Lela stared at the textbook on her desk. She
understood the fragment practice that Professor Markham was reviewing in
class. But was too shy to raise her hand to volunteer an answer.
You chose the second part, She understood the fragment practice that Professor Markham was reviewing in class. This part has a main clause: She is the subject; understood is the verb. A fragment will not have a main clause.