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A buffer system is a solution that withstands changes in $pH$ when an acid or base is added. The acidic solution contains a high concentration of hydrogen ions $(H+)$and has a $pH$ value of less than $7$. The basic solution contains a high concentration of hydroxide ions $\left(OH\right)$ and has a pH value above $7.$

a.

The solution of strong base and its salt is completely ionized. Therefore, the $NaOH$ and $NaCl$ solutions are not buffer systems.

b.

The mixture contains weak acid $H_2{CO}_3$ and its salt ${\mathrm{NaHCO}}_3$. Therefore, the solution of $H_2{CO}_3$and ${\mathrm{NaHCO}}_3$ is a buffer system.

c.

The mixture contains the weak acid $HF$ and its salt $KF.$ Therefore, the mixture of $HF$ and $KF$ is a buffer system.

d.

Both substances are salts. The mixture does not contain weak acids. Therefore, a mixture of $KCl$ and $NaCl$ is not a buffer.