Which statement must be true about a chemical system at equilibrium?
(1) The forward and reverse reactions stop.
(2) The concentration of reactants and products are equal.
(3) The rate of the forward reaction is equal to the rate of the reverse reaction.
(4) The number of moles of reactants is equal to the number of moles of product.



Answer :

(1) is wrong because at equilibrium both the reactant and the product are still being created they are just being created at the same rate meaning that

(3) is the answer. As the concentrations do not change, but the solution/system is still in dynamic equilibrium. 
Homewk
I agree with the answer below (partial credit would go to her/him I guess) 
My answer I guess is more visual to explain 
1 says --> | <--   
2 is assuming because not all chemical system are the same although this could be true in some cases
3 says     -->   which is true because that's the def of Equilibrium
              <-- 
4-I don't get that one-sorry