To convert 12g of ${\mathrm{NaH}}_2{\mathrm{PO}}_4$ completely into ${\mathrm{Na}}_3{\mathrm{PO}}_4$, the volume of 1 molar NaOH required is ${\mathrm{cm}}^3$ (nearest integer)

${\mathrm{NaH}}_2{\mathrm{PO}}_4( \mathrm{s})+2\mathrm{NaOH}\left(\mathrm{aq}\right)\to {\mathrm{Na}}_3{\mathrm{PO}}_4\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$
Moles of ${\mathrm{NaH}}_2{\mathrm{PO}}_4=\frac{12}{120}=0.1$ moles
0.1 moles of ${\mathrm{NaH}}_2{\mathrm{PO}}_4$ required 0.2 moles of NaOH.

Given that there is 1 mole of NaOH in 1000 ml of solution.
Consequently, 0.2 moles of NaOH $\to 0.2\times 1000$.
So, 200 ml is the required volume.

Hence, the correct answer is 200 ml.