Take a look at the passage again:
Maria wasn't 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 wasn't 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.
Go back to the passage to try again.