Answer is: 4. the forward and reverse reaction rates are the same.
The reaction rate is the speed at which reactants are converted into products.
The formation constant can be calculated when chemical equilibrium is reached, when the forward reaction rate is equal to the reverse reaction rate.
In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present in concentrations which have no further tendency to change with time.
Reaction rates of the forward and backward reactions are equal and there are no changes in the concentrations of the reactants and products.