The correct option is C Rennin
Rennin's primary function is to coagulate the milk. The stomachs of infants are not properly developed. That is why they cannot digest solid food in the first months of life. Also, because the digestive tract is very sensible, it might eliminate the milk young children usually ingest. Therefore, its very important that milk stays in the stomach and within the body as much as possible. This doesnt happen, when the milk is in its liquid form, but when its curdled. Thats the reason, why rennin is that essential for babies.