Saliva plays an important role in the initial process of digestion. It contains an enzyme called amylase, which aids in the breakdown of starches into simpler sugars. This process is known as starch digestion. Therefore, option 2 (starch) is correct.

Option 1, fats, is not digested in the mouth but rather in the small intestine with the help of bile and pancreatic enzymes.

Option 3, proteins, are primarily broken down in the stomach by the enzyme pepsin, not in the mouth.

Option 4, vitamins, are not directly digested by saliva. Vitamins are absorbed in the small intestine after being broken down by other digestive processes.

In summary, saliva assists in the digestion of starches through the action of amylase.