Casein form . . . . . . . . of total milk proteins.

Answer: Option D

Casein forms 80 - 83% of total milk proteins.
Casein is a family of related phosphoproteins (αS1, aS2, β, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of the proteins in human milk. Sheep and Cow milk have a higher casein content than other types of milk with human milk having a particularly low casein content.