Final answer:
The enzyme present in the saliva of young nursing mammals for the digestion of mother's milk is rennin.
Step-by-step explanation:
The enzyme that is found in the saliva of some mammals while they are very young and still nursing is d) Rennin. This enzyme plays a crucial role in digesting mother's milk, especially by curdling the milk proteins, which slows the passage of milk through the stomach and intestines. This allows more time for the milk's nutrients to be absorbed. It's important to note that while lipase is also present in saliva and it helps with fat digestion, and amylase breaks down carbohydrates, rennin is specifically associated with young mammals and the digestion of milk.