Aqueous solution of HNO₃, KOH, CH₃COOH and CH₃COONa of identical concentration are provided. The pair(s) of solutions which form a buffer upon mixing is (are)

In which of the following mixed aqueous solutions, pH = pK_a at equilibrium?

1. 100 mL of 0.1 M CH₃COOH + 100 mL of 0.1 MCH₃COONa

(2) 100 mL of 0.1 MCH₃COOH + 50 mL of 0.1 M NaOH

(3) 100 mL of 0.1 M CH₃COOH + 100 mL of 0.1 M NaOH

(4) 100 mL of 0.1 MCH₃COOH + 100 mL of 0.1 MNH₃