We have that the correct answer is the last one. NH4NO3 is called ammonium nitrate and is the salt that appears from combining HNO3 (acid) with NH3 (ammonia, base). We have then that a solution of this type is a buffer solution because ammonia is a weak base and the salt contains its conjugate acid (definition of buffer solution). The 2nd example is wrong because both potassium acetate and Acetic Acid are acids; no conjugate base is involved. HCI and NaCl are pairs of conjugacy (HCI is an acid and NACI can be produced by NaOH and HCI), but HCI is a strong acid and NACI is a salt. Hence, it cannot be a buffer solution (weak acid needed). Weak acid or base means that it does not fully break down in ions when in water.
