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 second part, So Santana, the family beagle, snatched a chicken leg hanging over the edge. This part has a main clause: Santana is the subject; snatched is the verb. A fragment will not have a main clause.
Go back to the passage to try again.