Final answer:
Rugae are folds in the stomach wall, formed by the muscularis mucosae. Their function is to increase the surface area for digestion and to aid in mechanically breaking down food.
Step-by-step explanation:
The rugae are folds in the stomach wall. These undulating folds are formed by the muscularis mucosae, a thin layer of smooth muscle in the mucosa of the stomach and small intestine, which is in constant tension. The primary function of these rugae is to dramatically increase the surface area available for digestion and absorption, thus allowing the stomach to expand as food enters and to help mechanically break down food particles.
The stomach wall is composed of the same four layers found throughout most of the alimentary canal - the mucosa, submucosa, muscularis, and serosa - with specific adaptations. In addition to the typical circular and longitudinal smooth muscle layers of the muscularis, an inner oblique muscle layer is also present, giving the stomach its ability to churn and mix food.
Therefore, the answer to the student's question is B: Rugae are folds in the stomach wall.