Which one of the following is the correct pairing of the site of action and the substrate of rennin?

Chymosin also known as rennin, is a proteolytic enzyme synthesized by chief cells in the stomach. Its role is in digestion or coagulate milk in the stomach. The majority of milk protein is casein and there are four major types of casein molecules. The casein normally keeps the majority of milk protein soluble and prevents it from spontaneously coagulating.