The correct option is C Ca
In young mammals milk protein caseinogens is digested by the help of an enzyme called rennin. It is secreted as prorennin, which changes into active rennin in the presence of HCl. Rennin curdles milk protein caseinogens, which is hydrolysed into casein. Rennin converts casein into solid calcium paracaseinate in presence of Ca++, so that pepsin can reduce it to proteoses.