Answer:
c The concentration(s) of reactant(s) is constant over time.
Step-by-step explanation:
When the reaction A ⇌ B reaches equilibrium, the concentrations of reactants and products are constant over time.
a is <em>wrong</em>, because the concentrations of reactants and products are usually quite different.
b is <em>wrong</em>, because both product and reactant molecules are being formed at equilibrium.
d is <em>wrong</em>. The rates of the forward and reverse reactions are equal, but they are not zero.