What is the state in which both reactants and products are present at concentrations that do not change over time?

The correct answer is the state known as chemical dynamic equilibrium. In this state, both reactants and products are present in a closed system, and their concentrations remain constant over time. This does not mean that the reactions have stopped; rather, the forward and reverse reactions occur at the same rate, leading to a balance where the concentrations of all species involved in the reaction do not change.

Dynamic equilibrium is a crucial concept in chemistry because it signifies that while the individual molecules are constantly reacting, the overall concentrations of the reactants and products stay stable.

In contrast, static equilibria would imply that there is no movement or change, which is not the case in chemical systems at equilibrium. Therefore, the terms static or physical in the other choices do not accurately describe the ongoing processes occurring at equilibrium.