Review the passage:
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 had her friends
at the table not mentioned her disarray?
You chose the third part, Why had her friends at the table not 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.