A vessel containing an equilibrium mixture of  $N{O}_2\left(g\right)$ and  $N_2{O}_4\left(g\right)$ at a certain temperature and 1.00 bar total pressure is represented below.

Which is the best representation of the system at equilibrium after a certain amount of argon is added to the vessel, while keeping both the temperature and the total pressure constant?
 

As some noble gas is added to a constant pressure system, the “net” pressure of the reactants and products actually decreases as the total pressure including the partial pressure of the noble gas keeps constant. According to Le Chatelier Principle, the equilibrium will shift to the decomposition side to produce more gas molecules (more $N{O}_2\left(g\right)$), which can reduce the pressure decrease. The only correct choice with more  $N{O}_2$ (less $N_2{O}_4$ ) is D. You may prove the answer quantitatively as well.