${\mathrm{MgSO}}_4$ on reaction with ${\mathrm{NH}}_4\mathrm{OH}$ with ${\mathrm{Na}}_2{\mathrm{HPO}}_4$ forms a white crystalline precipitate. What is its formula?

$\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4$ is white precipitate obtained when the magnesium sulphate is reacted with ammonium hydroxide and sodium hydrogen phosphate. The following reaction is obtained as the result

${\mathrm{MgSO}}_4+{\mathrm{Na}}_2{\mathrm{HPO}}_4+{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4+{\mathrm{Na}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}$

Hence, correct option is (a).