At equilibrium, which of the following statements is true regarding concentrations?

At equilibrium, the concentrations of reactants and products remain constant over time, although they may not necessarily be equal. This dynamic state indicates that the forward and reverse reactions occur at the same rate, leading to no net change in concentration.

While it might seem that concentrations could fluctuate, at equilibrium, any changes in the concentration of either reactants or products prompt the system to adjust and counter those changes, effectively maintaining constant levels. This understanding highlights the stability of an equilibrium state, where the rates of the reactants converting to products and the products reverting to reactants are balanced.

The other options imply that concentrations either specifically decrease or vary unpredictably, which contradicts the essence of equilibrium, where change is no longer observed in concentration over time. This reinforces why the correct assertion properly describes the nature of chemical systems at equilibrium.