Which of the following types exhibits 100\% atom economy?

Atom economy is the measure of the proportion of reactant atoms that become part of the desired product in a chemical reaction.

$\text{Atom economy} = \dfrac{\text{Molecular mass of desired product}}{\text{Molecular mass of all reactants}} \times 100$

In an addition reaction, two or more reactants combine to form a single product. Since no by-products are formed, all the atoms of the reactants are incorporated into the product.

Thus, addition reactions exhibit $100\%$ atom economy.

In substitution, elimination, and dehydrogenation reactions, by-products are formed, leading to an atom economy of less than $100\%$.

Answer: Addition