Final answer:
A reversible reaction is one that can proceed in both directions, where reactants become products that can revert back to reactants, eventually reaching chemical equilibrium, denoted by a double-headed arrow in equations (c).
Step-by-step explanation:
To describe a reaction as "reversible" means that the reaction can proceed in both the forward and backward directions. In such a reversible reaction, the reactants can be converted into products, and when the product concentration reaches a certain threshold, the reaction can reverse, converting the products back into reactants.
This process continues until a state of chemical equilibrium is reached, where the forward and reverse reactions both occur at the same rate, and the concentrations of reactants and products remain constant over time. Reversible reactions are often denoted by a double-headed arrow in chemical equations, pointing towards both the reactants and the products.