For  ${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3$, rates of disappearance of ${\mathrm{N}}_2$ and  ${\mathrm{H}}_2$ and rate of appearance of ${\mathrm{NH}}_3$ respectively are a, b and c, then

For every '1' mole of ${\mathrm{N}}_2$ disappearing, '3' moles of ${\mathrm{H}}_2$ disappears and '2' moles of ${\mathrm{NH}}_3$ forms.

This indicates rate of disappearance ${\mathrm{N}}_2$ < Rate of appearance of ${\mathrm{NH}}_3$ < Rate of disappearance of ${\mathrm{H}}_2$

a < c < b