Take a look at the passage again:
Horrified, Sherri stared at her reflection in the mirror. Hair sticking
out in seventeen different directions, a spot of whipped cream on her chin,
and a green pen mark across her nose. Why hadn't
her friends at the table mentioned her disarray?
You chose the third part, Why hadn't her friends at the table mentioned her disarray? This part has a main clause: friends is the subject; had mentioned is the verb. A fragment will not have a main clause. Remember also that a question is not necessarily a fragment.
Go back to the passage to try again.