Which is true when a reaction has reached equalibrium

The answer should be: D. The reaction rate is equal in both directions

In the equilibrium state, the rate of reaction to the right is same as the reaction to the left. Because of this, the concentration of the reactant and product will be kept same.

It might seem like the reaction is stopped because there is no change in the concentration, but it wasn't. Adding a reactant or product will break the equilibrium state.