Take another look at the passage:
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 first part, As quietly as possible, Sherri tried to open the one-pound bag of candy .... This part has a main clause: Sherri is the subject; tried is the verb. A fragment will not have a main clause.