Which reaction is an example of an oxidation-reduction reaction?

1. AgNO₃+KI→AgI+KNO₃
2. Cu+2AgNO₃→Cu(NO₃)₂+2Ag
3. 2KOH+H₂SO₄→K₂SO₄+2H₂O
4. Ba(OH)₂+2HCl→BaCl₂+2H₂O