_________ is an example of a substitution reaction. 

Substitution reaction is the substitution of the functional group of one chemical compound by another group. It involves the replacement of an atom or group of atoms which are present in a compound by some other atom or group of atoms. Halogenation is the replacement of a hydrogen atom by a halogen atom in a molecule. When halogen reacts with alkanes under the presence of heat and light, they form alkyl halides. 

${\mathrm{CH}}_4+{\mathrm{Cl}}_2\to {\mathrm{CH}}_3\mathrm{Cl} + \mathrm{HCl}$

Combination, Hydrogenation, and Hydration are examples of the addition reaction.

Hence, Halogenation is an example of a substitution reaction.