The content of the part labelled "III" is usually acidic. In the diagram, "III" refers to the stomach, which is a part of the digestive system. The stomach is a muscular sac that contains digestive enzymes and hydrochloric acid. The acid helps to break down food and kill any harmful bacteria that may be present. The pH level of the stomach acid is usually between 1.5 and 3.5, which is highly acidic. This acidic environment is necessary for the proper digestion of food and the absorption of nutrients. Therefore, the content of the stomach is usually acidic.