How Is the Reaction Quotient Used to Determine Whether a System

How is the reaction quotient used to determine whether a system is at equilibrium?

A) The reaction quotient must be satisfied for equilibrium to be achieved.
B) At equilibrium, the reaction quotient is undefined.
C) The reaction is at equilibrium when Q < K_eq.
D) The reaction is at equilibrium when Q > K_eq.
E) The reaction is at equilibrium when Q = K_eq.