Which of the following is an acid salt ?

${\mathrm{Na}}_3{\mathrm{PO}}_4$ is a neutral salt because $3\hspace{0.17em}\mathrm{NaOH}+{\mathrm{H}}_3{\mathrm{PO}}_4\to {\mathrm{Na}}_3{\mathrm{PO}}_4+3{\mathrm{H}}_2\mathrm{O}.$

${\mathrm{Na}}_2{\mathrm{H}}_2{\mathrm{PO}}_4$ is an acidic salt where 2 hydrogen’s can be replaced by Na in the structure; $\mathrm{NaOH}+{\mathrm{H}}_3{\mathrm{PO}}_4\to {\mathrm{NaH}}_2{\mathrm{PO}}_4+{\mathrm{H}}_2\mathrm{O} \mathrm{others} {\mathrm{Na}}_2{\mathrm{HPO}}_3 \mathrm{and} {\mathrm{NaH}}_2{\mathrm{PO}}_2$neutral salt.

structure of ${\mathrm{NaH}}_2{\mathrm{PO}}_4$            Acidic salt is ${\mathrm{NaH}}_2{\mathrm{PO}}_4$.