Explanation:

- Aggravation: This term means making something worse or more severe. It is the opposite of what we are looking for.

- Mediation: This refers to the process of intervening in a dispute to resolve it. It does not necessarily relate to reducing severity but to facilitating agreement or resolution.

- Palliation: This means making something less severe or intense without removing the cause. It's often used in a medical context to relieve symptoms.

- This is the correct answer for making something less severe.

- Reduction: This involves making something smaller in size or amount. It doesn't specifically denote the context of making something less severe.