(b)
Considering the given reaction,
A total of 2 mol of gas is present on the reactant side, and a total of 4 mol of gas is present on the product side.
As the volume of the system increases, the pressure decreases, allowing more "space" for our gas molecules. In this case, the reaction will produce more gas, which can only happen if the reaction is shifted to the side with more gas molecules.
In comparison to the reactant side, the product side has more gas molecules. As the reaction progresses to the right, the amount of product produced will increase while the amount of reactant consumed will decrease.
Therefore, both the reactants and will decrease, and both products and will increase.