Take a look at the passage again:
Diane picked at the slice of cheesecake with her car keys. While Nick sucked
whipped cream off his fingers. Horrified, Rachel
watched her roommates decimate the dessert that she had spent three hours
preparing.
You chose the third part, Horrified, Rachel watched her roommates decimate the dessert .... This part has a main clause: Rachel is the subject; watched is the verb. A fragment will not have a main clause.
Go back to the passage to try again.