The answer to the blank in the question is concentration or amount. I would just like to introduce here the concept of Le Chatelier's Principle to explain to you how I know the answer. Le Chatelier's Principle is applied to equilibrium reactions written as
A + B ⇆ C + D
An equilibrium reaction has two reactions that are occurring in dynamic equilibrium. These are the forward and reverse reactions, denoted by the symbol ⇆. We can manipulate the direction of the reaction by adding the concentration or amount of either the reactants or products, and by changing the temperature or the pressure. The principle states that once the reaction is disturbed by incorporation the changes mentioned earlier, the reaction attempts to regain equilibrium by minimizing the effect of the disturbance. So, if you add the concentration of the products, it will favor the forward reaction to produce more products. In this way, it will come to equilibrium once again. The same is true for temperature depending on the heat of reaction, or for pressure depending on reactions involving gases.
