In case of the decomposition of hydroiodic acid (2H$$I$$ ⇋ H₂ + $${{I_2}}$$), addition of H₂ (at equilibrium condition) will

Lechalters principle: Actually for an equilibrium reaction on increasing the reactant concentration the equilibrium shifts towards the right and therefore the reactants gets more dissociated and products concentration increases but here we are increasing the product $$\left( {{H_2}} \right)$$  concentration so the equilibrium shifts towards left and increases the formation of $$2HI$$  there by decreasing the dissociation of $$HI$$.