In which one of the given reactions a salt is reacting with a base?

Q. In which one of the given reactions a salt is reacting with a base?

1. NaOH+HCl→NaCl+H₂O
2. Cu(OH)₂+H₂SO₄→CuSO₄+2H₂O
3. KOH+HCl→KCl+H₂O
4. 6NH₄OH+Al(SO₄)₃→2Al(OH)₃+3(NH₄)₂SO₄

Answer: 6NH₄OH+Al(SO₄)₃→2Al(OH)₃+3(NH₄)₂SO₄