The enzyme that coagulates milk into curd is option 1, Rennin. Rennin is a protease enzyme found in the stomachs of young mammals, particularly in the lining of their fourth stomach compartment called the abomasum. Rennin plays a crucial role in the digestion of milk protein known as casein. It works by breaking down the peptide bonds in casein, causing the milk to coagulate and form curd. This process is essential for the digestion and absorption of nutrients from milk in young animals. Pepsin, option 2, is another enzyme that aids in protein digestion, but it does not have the specific ability to coagulate milk. Resin, option 3, and citrate, option 4, are not enzymes and are not involved in the coagulation of milk.