The correct answer is (C) To strengthen; to toughen or temper. To heat and gradually cool glass, earthenware, metals, etc., to remove or prevent internal stress and prevent brittleness.

Anneal comes from Middle English anelein, Old English anælan, equivalent to an+ælan (burn, bake).

Examples of use:

• His years of service served to anneal his loyalty.
• Steel is annealed after the rolling operation in the steelmaking process.