The correct option is A Mouth, Stomach, Duodenum
Since Kevin suffered indigestion by eating carbohydrates and proteins, digestion might have gone wrong in the places where carbohydrates and proteins are broken down into smaller molecules.
The process of digestion first starts in mouth where few carbohydrates are broken down by salivary amylase into simple sugars (disaccharides). There is no digestion of carbohydrates in stomach. Pancreatic amylase and Intestinal juice further breaks disaccharides into monosaccharides in duodenum and ileum.
Protein digestion starts in the stomach. Gastric juice breaks protein molecules into peptides. Peptides are further broken down by pancreatic and intestinal juice in the duodenum and the ileum.
So the indigestion might have occurred in mouth, stomach, duodenum and ileum.