How does the addition of an inert gas at constant volume affect a reversible reaction at equilibrium?

The addition of an inert gas at constant volume does not alter the position of equilibrium because it does not participate in the reaction. When an inert gas is introduced into the system without changing the volume, it increases the total pressure of the system, but the partial pressures of the reactants and products remain unchanged. Equilibrium depends on the concentrations (or partial pressures) of the reactants and products, so if those are constant, the equilibrium position remains stable.

While some of the other choices suggest changes in pressure or temperature, the essential point is that inert gases do not react or affect the established concentrations of reactants and products in a reversible reaction, leading to the conclusion that the equilibrium remains unaffected.