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

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

Nitrogen combines with hydrogen to generate more ammonia, therefore increasing the quantity of nitrogen must inevitably result in a decrease in the concentration of hydrogen. Nitrogen reacts with hydrogen to make more ammonia, so the concentration of ammonia must inevitably grow.

Hence option A is correct