Take a look at the passage again:
Cringing with anxiety at the grade that she might find.
Nancy peeked at the last page of the essay.
The big red A sent waves of relief radiating through her body.
You chose the second part, Nancy peeked at the last page of the essay. This part has a main clause: Nancy is the subject; peeked is the verb. A fragment will not have a main clause.
Go back to the passage to try again.