${\mathrm{NH}}_3$ gas is liquefied more easily than ${\mathrm{N}}_2.$ Hence -

Van der Waals constants a and b of ${\mathrm{NH}}_3>\mathrm{that} \mathrm{of} {\mathrm{N}}_2$

$\mathrm{a}\left({\mathrm{NH}}_3\right)>\mathrm{a}\left({\mathrm{N}}_2\right) \mathrm{but} \mathrm{b}\left({\mathrm{NH}}_3\right)<\mathrm{b}\left({\mathrm{N}}_2\right)$

Van der Waals constants a and b of ${\mathrm{NH}}_3<\mathrm{that} \mathrm{of} {\mathrm{N}}_2$

$\mathrm{a}\left({\mathrm{NH}}_3\right)<\mathrm{a}\left({\mathrm{N}}_2\right) \mathrm{but} \mathrm{b}\left({\mathrm{NH}}_3\right)>\mathrm{b}\left({\mathrm{N}}_2\right)$