The correct answer here is D. Starches are complex carbohydrates that are made up of multiple sugars. Amylase breaks down the carbohydrates into their constituent sugars which are naturally sweet tasting. A is factually correct but proteins break down into amino acids which are not sweet. B is incorrect (natural sugar in your mouth would be bad for teeth). C is factually correct but you would not taste any of it in your mouth. Finally, peristalsis in E involves muscle contractions to move food through the esophagus and has nothing to do with taste.