Review 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 Professor 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.