The correct option is C Rennin
In young mammals, milk protein caseinogen is digested with the help of an enzyme called rennin. Prorennin is converted into active rennin in the presence of HCl. Rennin curdles milk protein caseinogens, which is hydrolyzed into casein. Rennin converts casein into solid calcium paracaseinate in presence of Ca++, so that pepsin can reduce it to peptones. Hence, the correct answer is B.