Answer:
The fibers have lost electrons giving them a positive charge. The
rubber gained electrons giving it a negative charge.
Since electrons have a negative charge, the balloon now has a
negative charge. But the sweater fibers lost some of their electrons,
so now the fibers have a positive charge (they have more protons
than electrons). Positive and negative attract so if you bring the
balloon near the fibers, they move toward the balloon.
