The correct option is D Pepsin
Milk digestion begins in the acidic environment of the stomach, where pepsin starts protein digestion by breaking down milk proteins into smaller fragments.
Lingual lipase does the same to milk fats. These smaller fragments then move into the small intestine for further digestion.
In humans, milk protein-digesting enzyme in the stomach is pepsin. In calves it is rennin. Rennin is also present in small amounts in human infants but not adults.
Pepsin acts on water soluble caseinogen (milk protein) to form solubles 'casein'. This combines with calcium salts to form insoluble calcium paracaseinate, which gets readily digested enzymatically.
Thus the correct answer is option D.