The correct option is C Rennin
The enzyme rennin is found only in infants. Its role in digestion is to curdle or coagulate milk in the stomach. So rennin is the enzyme which specifically acts on milk proteins(casein).In adults, pepsin is an enzyme that breaks down proteins into smaller peptides and not specifically casein. So rennin is the most appropriate option.
Trypsin is the enzyme that is synthesized by the pancreas and secreted into the small intestine and not the stomach.
Lipase is a pancreatic enzyme that catalyzes the breakdown of fats to fatty acids and glycerol or other alcohols.