Final answer:
Cooling a reaction mixture at equilibrium shifts the equilibrium to the product side in an exothermic reaction because this reaction type releases heat as a product. Such a shift counteracts the decrease in temperature by producing more heat.
Step-by-step explanation:
The student's question suggests that if cooling a reaction mixture at equilibrium shifts the equilibrium to the product side, then the reaction must be characterized by one of the given options. The correct answer is d. exothermic.
An exothermic reaction is one that releases heat as a product. When the temperature is decreased, the system attempts to counter this change by shifting the equilibrium toward the heat-producing direction, thus forming more products.
In contrast, an endothermic reaction would shift towards the reactants when cooled, as these reactions absorb heat from their surroundings. Examples of exothermic reactions include combustion, where heat is given off, and the process of freezing water, where heat is removed from the water resulting in ice formation.