Adding an inert gas at constant volume decreases the partial pressures of the reactants, thereby shifting the equilibrium to the right.

Adding an inert gas at constant volume does not change the partial pressures of the reacting gases, so the equilibrium position remains unchanged.

Adding an inert gas at constant volume increases the total pressure and shifts the equilibrium to the side with fewer moles of gas.

Adding an inert gas at constant volume dramatically alters the kinetic energy of the molecules, forcing a shift to the left.