(C) starch into simple sugars
Saliva contains the enzyme salivary amylase which breaks down starch into simple sugars. Less saliva means less amylase, so starch digestion in the mouth is affected.
Source: Life Processes, Section 5.2.4
The passage clearly states: "The saliva contains an enzyme called salivary amylase that breaks down starch… to give simple sugar." Proteins are digested by pepsin (stomach) and trypsin (small intestine); fats by lipase (small intestine). Option (D) is fermentation by yeast — not a digestive process. Examiners expect you to directly link saliva → salivary amylase → starch digestion.