Which of the following is NOT a type of chemical reaction?

A monopolar reaction is not recognized as a type of chemical reaction in standard chemistry terminology. The accepted classifications of chemical reactions primarily include types like combination reactions, where two or more substances combine to form a single product, decomposition reactions, where a single compound breaks down into two or more products, and double substitution reactions, where there is an exchange of ions between two compounds. Since monopolar reactions do not fit into these established categories, identifying it as the option that is not a recognized type of chemical reaction is accurate.