Take a look at the passage again:
As quietly as possible, Sherri tried to open the one-pound bag of candy
on her lap. Finally, the tough plastic split open.
Causing an explosion of chocolate balls that
bounced and rolled down the aisles of Dr. Wilson's chemistry class.
You chose the second part, Finally, the tough plastic split open. This part has a main clause: plastic is the subject; split is the verb. A fragment will not have a main clause.
Go back to the passage to try again.