For the reaction ${\mathrm{N}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\rightleftharpoons 2{\mathrm{NH}}_3\left(\mathrm{g}\right)+93.6\mathrm{kJ},\mathrm{t}$ the concentration of  ${\mathrm{H}}_2$, at equilibrium will increase if

For the reaction ${\mathrm{N}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\rightleftharpoons 2{\mathrm{NH}}_3\left(\mathrm{g}\right)+93.6\mathrm{kJ},\mathrm{t}$ the concentration of  ${\mathrm{H}}_2$, at equilibrium will increase if the volume is decreased

As a result, a gas-phase reaction with a smaller volume will cause the system to move toward the reaction's side with the fewest gas molecules. For instance, if the volume of the subsequent reaction is reduced, the system will be shifted toward more products since the reactants and products would be concentrated more tightly at equilibrium.

Hence option B is correct