Answer: D. neutralization
Explanation:
1. Substitution reaction is defined as the reaction in which one atom replaces the other atom from the compound.

2. Saponification is a reaction in which fatty acids are made to react with alkalis to form sodium salt of fatty acids called as soaps.
3. Ionization is a type of chemical reaction in which a substance ionizes when dissolved in water.

4. Neutralization is a type of chemical reaction in which an acid reacts with base to produce salt and water.