Given that ${\mathrm{N}}_{2\left(g\right)}+3{\mathrm{H}}_{2\left(\mathrm{ }\mathrm{g}\right)}⟶2{\mathrm{NH}}_{3\left(\mathrm{ }\mathrm{g}\right)};\Delta \mathrm{H}=-92\mathrm{KJ}$ the standard molar enthalpy of formation in $\mathrm{kJ}{\mathrm{mol}}^{-1}$ of ${\mathrm{NH}}_3\left(\mathrm{ }\mathrm{g}\right)$ is

 Reacting 1 mole nitrogen with 3 mole hydrogen produces 2 moles of ammonia with the heat of formation $-92\mathrm{KJ}$

 $\Delta H_f=-92\mathrm{KJ}$ for 2 moles of ammonia

 Then for one-mole ammonia the standard molar enthalpy is

 $\Delta H_f^0=-\frac{92}{2}=-46\mathrm{ }\mathrm{K}.\mathrm{cal}$

Hence the correct option is (D).