Final answer:
The correct option is c. The circular folds in the small intestine, known as plicae circulares, do not flatten out when the organ stretches and are essential for nutrient absorption. Unlike these, the mucosal folds in other parts of the gastrointestinal tract can flatten out to accommodate stretching.
Step-by-step explanation:
The question involves understanding the structural features of different parts of the gastrointestinal tract and how they respond when the respective organ stretches. The circular folds (plicae circulares) in the small intestine are unique in that they do not flatten out when the organ stretches. These folds are deep ridges in the mucosa and submucosa which facilitate absorption by causing the chyme to spiral through the small intestine, thus slowing its movement and providing more time for nutrient absorption.
Other mucosal folds such as the rugae in both the gallbladder and the stomach, as well as the longitudinal folds in the esophagus, are capable of flattening out as these organs stretch. The muscularis mucosae layer maintains a constant state of tension that pulls the mucosa into undulating folds, which increases the surface area for digestion and absorption.
Therefore, the correct option is c. circular folds (plicae circulares) in the small intestine.