A solution is 0.1 M in ${\mathrm{CH}}_3\mathrm{COOH}$ and 0.1 M in ${\mathrm{CH}}_3\mathrm{COON} \mathrm{a}$. Which of the following will change its pH significantly?

Buffer solution is a solution that does not change its pH value when a weak acid or basic is added (less than 1%), or when the solution is diluted.
Buffer action is the process by which a buffer solution works, while buffer capacity is the ability of a solution to resist changes in pH value.
buffer solution with acid A weak acid and its salt are combined with a strong base to form an acidic buffer solution.
Here, the acid and salt concentrations in the buffer solution are the same.

$\mathrm{pH}={\mathrm{pK}}_{\mathrm{a}}+\log \frac{\left[\text{ salt }\right]}{\left[\text{ acid }\right]}$

pH will remain constant as long as the volume of the solution stays constant.