Concept:The first site where protein digestion begins is the stomach, due to the action of pepsin.Explanation:In the diagram of the farm animal's digestive tract, part III represents the stomach.The stomach wall secretes hydrochloric acid and pepsinogen.Hydrochloric acid makes the medium acidic and converts pepsinogen into active pepsin.Pepsin starts breaking down large protein molecules into smaller peptides.Thus, protein digestion begins in the stomach, labelled as III.Answer:C. III