Concept:For a chemical reaction to start, existing bonds in the reactant molecules must first be broken.Explanation:Reactant particles must collide with sufficient energy, called activation energy.This energy is used to break the chemical bonds present in the reactants.Only after these bonds break can new bonds form to create the products.Therefore, the essential condition before a reaction can take place is the breakage of bonds of reactants.Answer:C. breakage of bonds of reactants