The correct option is A Rennin
Gastric juice consists of gastric enzymes like pepsinogen, pro rennin, gastric lipase etc. Rennin is active in the childhood stage of mammals only. It converts milk into curd like substance (called clotting of milk) and then digests it. In adult stages, it is inactive. Rennin, acts on milk protein caseinogen. Caseinogen is a soluble protein. In presence of rennin, caseinogen gets converted into insoluble caseinate. This process is termed as curdling of milk. After becoming insoluble, milk can remain in the stomach for a longer time. Rennin is absent in human and clotting of milk is done by hydrochloric acid in humans.