A student needs to prepare a buffer solution of propanoic acid and its sodium salt with $\mathrm{pH}4$. The ratio of $\frac{\left[{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{COO}}^{-}\right]}{\left[{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{COOH}\right]}$ required to make buffer is Given : ${\mathrm{K}}_{\mathrm{a}}\left({\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{COOH}\right)=1.3\times {10}^{-5}$

• A

  $0.03$
• Correct answer
  B

  $0.13$
• C

  $0.23$
• D

  $0.33$

Hence, the correct answer is option: 'B'.