Which can act as buffer :-
(1) NH₄OH + NaOH
(2) HCOOH + CH₃COONa
(3) 40 mL 0.1 M NaCN + 20 mL of 0.1 M HCl
(4) None of them

The solution is as follows:

Correct option is (2) HCOOH + CH₃COONa.


This is because, the sodium acetate completely dissociates in the solution as shown below:

              CH_​3COONa $\to$ CH₃COO^- + Na⁺

this supresses the ionisation of HCOOH and it acts as a buffer solution.