Which of these is an indicator of a reaction at equilibrium?

The indication of a reaction at equilibrium is when the forward and reverse reactions occur at equal rates. This means that the rate at which reactants are converted into products matches the rate at which products are converted back into reactants. As a result, the concentrations of both reactants and products remain constant over time, although they may not be equal in amount. This balance reflects a dynamic state where both reactions are still occurring, but there is no net change in concentration.

In contrast, if reactants are completely consumed or if all reactants are converted to products, those scenarios would indicate that the reaction has gone to completion and is not at equilibrium. The reaction stopping entirely signifies that there is no further reaction occurring, which also indicates a lack of dynamic balance characteristic of an equilibrium state.