Process of adding vitamins to milk is known as:

The correct answer is Fortification.

• Food fortification is the process of adding micronutrients (essential trace elements and vitamins) to food.
• It is the practice of deliberately increasing the content of an essential micronutrient, i.e. vitamins and minerals (including trace elements) in a food.
• It is done so as to improve the nutritional quality of the food supply and provide a public health benefit with minimal risk to health.

 Additional Information

• Flavoring is the extracts, essences, and flavors that are added to foods to enhance their taste and aroma. 
• Fermentation is a chemical process by which molecules such as glucose are broken down anaerobically (Absence of Oxygen).
• Sterilization refers to any process that removes, kills, or deactivates all forms of life (in particular referring to microorganisms such as fungi, bacteria, viruses, spores, unicellular eukaryotic organisms such as Plasmodium, etc.).