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 first part, Diane picked at the slice of cheesecake with her car keys. This part has a main clause: Diane is the subject; picked is the verb. A fragment will not have a main clause.
Go back to the passage to try again.