Assertion : In Haber's process,${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ combine in 1 : 3 volume ratio 

Reason : Gases combine in a simple volume ratio

${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3$

In this equation, 1 volume of nitrogen gas combines with 3 volumes of hydrogen gas to give 2 volumes of ammonia gas. Thus the ratio of ${\mathrm{N}}_2: {\mathrm{H}}_2$ is 1:3, which is a simple ratio.

Hence, both the statements are true and R is the correct explanation of A.