When mixture is cooled, equilibrium will shift to left. Is this reaction endothermic or exothermic

Equilibrium reaction 3O2<=>2O3
A. Endothermic; B. Exothermic

This question pertains to Le Chatlier's principle. If the equilibrium of this reaction shifts left, therefore the reactants are less than the products. By examining the transition state on a diagram of this species, one can conclude that it is endothermic because the products are greater than the reactants.

Endothermic