Review the passage:
Maria was not watching her plate of barbecue very carefully.
So Santana, the family beagle, snatched a chicken leg hanging over the edge.
As baked beans and potato salad slid onto Maria's new sandals.
You chose the first part, Maria was not watching her plate of barbecue very carefully. This part has a main clause: Maria is the subject; was watching is the verb. A fragment will not have a main clause.