The correct option is C rennin – casein
Rennin is the enzyme that acts on casein which is a milk protein. It is secreted by the gastric glands in an inactive form prorennin which is activated by HCI. Rennin converts caesinogen into calcium paracaesinate in the presence of calcium ions. This ensures that milk remains in the stomach as long as possible. The largest amounts of rennin are present in the stomach of young mammals. Carbohydrate, maltose and protein are acted upon by carbohydrase, maltase and protease respectively.