To solve this problem, we need to identify the enzyme responsible for the initial step in the digestion of milk in humans.$\newline$Let's analyze each option:$\newline \bullet$Lipase:$\newline$ - Lipase is an enzyme that breaks down fats into fatty acids and glycerol.$\newline$ - It is not involved in the initial digestion of milk proteins.$\newline \bullet$Trypsin:$\newline$ - Trypsin is a protease enzyme that digests proteins in the small intestine.$\newline$ - It is not responsible for the initial digestion of milk.$\newline \bullet$Rennin:$\newline$ - Rennin, also known as chymosin, is an enzyme that curdles milk by coagulating its proteins, particularly casein.$\newline$ - It is the enzyme responsible for the initial digestion of milk in infants.$\newline \bullet$Pepsin:$\newline$ - Pepsin is a protease enzyme that digests proteins in the stomach.$\newline$ - While it aids in protein digestion, it is not specific to milk.$\newline$Based on the analysis, the correct answer is Option 3: Rennin, as it is responsible for the initial step in the digestion of milk in humans.