Haber’s process ${\mathrm{N}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\rightleftharpoons 2\mathrm{N}{\mathrm{H}}_3\left(\mathrm{g}\right)$

According to the reaction,

2 moles of ammonia are produced from 3 moles of ${\mathrm{H}}_2$

i.e., 1 mole of ammonia is produced from $\frac{3}{2}$ moles of ${\mathrm{H}}_2$

Thus, 20 moles of ammonia $=\frac{3}{2}\times 20=30$ moles of ${\mathrm{H}}_2$

Hence, 30 moles is the correct option.