An exothermic reaction is one in which the reacting substances:

Explanation

In an exothermic reaction, the reactants have more stored chemical energy than the products. This excess energy is released into the surroundings, usually in the form of heat, leading to an increase in the temperature of the surroundings. Option A is correct because it accurately describes this energy relationship. Option B is incorrect because it describes an endothermic reaction, where energy is absorbed. Option C is incorrect because identical energy levels would result in no net energy change. Option D is incorrect because it confuses temperature with energy content; while an exothermic reaction can release energy that increases temperature, the key concept is the energy difference between reactants and products.