Disclaimer: I haven't taken the MCAT yet or even started studying (but I am in Ochem2 and have done very well in chemistry as a whole) and am not 100% on my answer.
There are more moles of the products in that equation than in the reactants. So if you increase the size of the flask, we would get more products. This only has to do with gases, and so answer C, which might seem like it qualifies, does not qualify because there are 6 moles of gas on either side.