Explanation:
The correct answer is:

A. renin

Renin (also known as chymosin) is an enzyme that hydrolyzes milk protein, specifically casein, in the stomach of young mammals. It plays a critical role in the digestion of milk by curdling it.

The other enzymes listed serve different functions:
- Amylase breaks down carbohydrates.
- Lipase breaks down fats.
- Lactase breaks down lactose (a sugar in milk).
- Pepsin breaks down proteins but is not specific to milk proteins like renin.