How is the reaction quotient used to determine whether a system is at equilibrium? a at equilibrium, the reaction quotient is undefined. b the reaction is at equilibrium when q> keq. c the reaction is at equilibrium when q=keq. d the reaction is at equilibrium when q

The correct answer is C the reaction is at equilibrium when q = Keq
1-As when q > Keq :
the reaction is not at equilibrium, the reaction will move towards the left to increase the reactants and decrease the products to achieve equilibrium, and the reaction is preferring the reverse reaction to achieve the equilibrium.

2- and when q< Keq:
the reaction is not at equilibrium, the reaction will move towards the right to increase the products and decrease the reactants to achieve equilibrium, and the reaction is preferring the forward reaction to achieve the equilibrium.

3-and when q= Keq
This means that the reaction is at equilibrium, and there is no shift to right or to the left to make equilibrium, and the forward and reverse reaction is at the same rate. 
So the correct answer is C the reaction is at equilibrium when q = Keq

It changes food into energy that the organism can use.