Equilibrium is established in a reversible reaction when: a) the [product] = [reactants] b) rate of reaction of products = rate of reaction of reactants c) all the reactants dissolve or dissociate d) when product are no longer produced

The correct option is A.
A reversible reaction is one in which the reactants form a products, which can react back to form the reactants. Thus, a reversible reaction can proceed in both directions; forward and backward. Equilibrium is said to be attained when the rate of forward reaction equal the rate of backward reaction. All the reactants and products concentrations are constant at equilibrium.