The value of $\text{'}a\text{'}$ for ${\mathrm{NH}}_3$ is greater than that of ${\mathrm{N}}_2$

The value of van der Waal's constant, $\text{'}a\text{'}$ is greater for ${\mathrm{NH}}_3$ than for ${\mathrm{N}}_2$ due to the stronger attraction forces in ammonia (H-bonding).

Thus, the correct answer is B.