Final answer:
According to LeChatelier's principle, when the reaction quotient is less than the equilibrium constant, the reaction must shift toward products to reach equilibrium. This happens because the system seeks to increase the reaction quotient until it equals the equilibrium constant, thus reinstating equilibrium. The correct option is A.
Step-by-step explanation:
According to LeChatelier's principle, when the reaction quotient (Q) is less than the equilibrium constant (K) for a reaction, the system is not in equilibrium and will shift to re-establish equilibrium. In this scenario, the concentration of products is lower than necessary for equilibrium, as indicated by a Q value that is less than the K value. Therefore, to attain equilibrium, the reaction will shift in the direction that increases the Q value by forming more products until the value of Q rises to equal K.
Thus, when Q < K, the reaction shifts toward the products to reach equilibrium. This shift will continue until the concentrations of reactants and products have changed such that Q equals K again.
Options A and B specifically refer to the direction in which the reaction will shift, while options C and D describe changes in standard enthalpy of formation which is not directly related to the adjustment of the reaction according to Le Chatelier's principle. As the correct response is about the direction of the shift required to re-establish equilibrium, the correct option is A: the reaction must shift toward products to reach equilibrium.