For reaction, ${\mathrm{N}}_2( \mathrm{g})+3{\mathrm{H}}_2( \mathrm{g})\rightleftharpoons 2{\mathrm{NH}}_3( \mathrm{g})$ ,following graph is obtained:

The change applied at t = 40 sec. is possibly:

Given : $\left[{\mathrm{H}}_2\right]$ decreases suddenly & increases at $t=40s$

Reaction: ${\mathrm{N}}_2( \mathrm{g})+3{\mathrm{H}}_2( \mathrm{g})\iff 2{\mathrm{NH}}_3( \mathrm{g})$

(1): Adding $\left[{\mathrm{H}}_2\right]$ (reactant) shifts equilibrium to the right, decreasing $\left[{\mathrm{H}}_2\right]$

$\Rightarrow$Not correct

(2): $△\mathrm{n}=-2$, Increasing temperature shifts equilibrium to left, increasing $\left[{\mathrm{H}}_2\right]$ but sudden not explaines

$\Rightarrow$Not Correct

(3): Catalyst only alters the speed of reaction 

$\Rightarrow$Not Correct

(4): Pressure decreases suddenly $\Rightarrow$Volume increased suddenly without attaining equilibrium

[Products] & [Reactants] both decrease

Then later it attains equilibrium shifting reaction to left increasing $\left[H_2\right]$

$\Rightarrow$Correct Option