Which of the following can be used as a buffer solution?

The $\mathrm{pH}$ of the resulting solution of$20 \mathrm{mL}$ of  $0.1\mathrm{M} {\mathrm{H}}_3{\mathrm{PO}}_4$ and $20\mathrm{ }\mathrm{mL}$ of $0.1\mathrm{M} {\mathrm{Na}}_3{\mathrm{PO}}_4$ is:

In a buffer solution consisting of ${\mathrm{NaH}}_2{\mathrm{PO}}_4$ and ${\mathrm{Na}}_2{\mathrm{HPO}}_4$

$\left(1\right) {\mathrm{NaH}}_2{\mathrm{PO}}_4$ is a acid and${\mathrm{Na}}_2{\mathrm{HPO}}_4$ is salt.

$\left(2\right)$ The $\mathrm{pH}$ of the solution can be calculated using the relation: $\mathrm{pH}={\mathrm{K}}_{{\mathrm{a}}_2}+{\log }_{10}\frac{\left[{\mathrm{HPO}}_4^{2-}\right]}{\left.{\mathrm{H}}_2{\mathrm{PO}}_4^{-}\right]}$.

$\left(3\right)$ The ${\mathrm{Na}}_2{\mathrm{HPO}}_4$ is acid and ${\mathrm{NaH}}_2{\mathrm{PO}}_4$ is salt.

$\left(4\right)$ The $\mathrm{pH}$ cannot be calculated.