Concept:A reaction that releases heat to the surroundings is called exothermic.Explanation:When magnesium burns in oxygen, it forms magnesium oxide.The reaction is: 2Mg+O2→2MgO.This reaction produces a bright light and releases a large amount of heat.Because heat is evolved, the reaction is exothermic.It is not endothermic because endothermic reactions absorb heat.Answer:Heat is evolved and the reaction is exothermic.Correct option: C.