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 third part, The big red A sent waves of relief radiating through her body. This part has a main clause: A is the subject; sent is the verb. A fragment will not have a main clause.
Go back to the passage to try again.