The reaction of two gases in a sealed container produces a different gas, and the system reaches equilibrium. what change in concentration would shift the equilibrium to the left?
decreasing the concentration of either a reactant or the product
increasing the concentration of either a reactant or the product decreasing the concentration of a reactant or increasing the concentration of the product
increasing the concentration of a reactant or decreasing the concentration of the product

The correct answer would be the third option. In order for the equilibrium of the reaction to shift to the left, we must decrease the concentration of a reactant or increase the concentration of the product. This is based on Le Chatelier's principle which describes the effect on equilibrium of changes in concentration or pressure of a product or a reactant. Adding more product in a system, would disturbed the equilibrium thus it would cause to reestablish it by shifting the reaction to the left producing the reactants. This would also be the same when you decrease the amount of reactant in the system.

The answer is b on plato.