The potential energy lost during the formation of product (B) is gained by the surroundings.
Exothermic reactions is a type of chemical reaction in which heat is lots to the surroundings.
From the given diagram, the energy of the reactants are more when compared to the products, so the reaction is exothermic and heat is released to surroundings.
Thus, the potential energy lost during the formation of product (B) is gained by the surroundings.
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