Which of the following best defines a reversible process in the context of chemical equilibrium?
a) A reaction that proceeds until the concentrations of reactants and products become equal.
b) Any transformation that involves no net energy change between reactants and products.
c) A process that can proceed in both the forward and reverse directions under the same conditions until the forward and reverse rates become equal.
d) A reaction that can proceed in both directions but only under different conditions (e.g., altered temperature or pressure).