Consider the reaction.

2 upper S upper O subscript 2 (g) plus upper O subscript 2 (g) double-headed arrow 2 upper S upper O subscript 3 (g).

When does the given chemical system reach dynamic equilibrium?

when the forward and reverse reactions stop
when the rate of the forward reaction is higher than the rate of the reverse reaction
when the concentration of the reactants is higher than the concentration of the products
when the rates of the forward and reverse reactions are equal