What structure is described as a fold in the mucosal membrane?

The term "ruga" refers specifically to folds or ridges in the mucosal membrane, which is commonly associated with structures like the stomach. These folds allow for the expansion of the organ as it fills with food or liquid. The presence of rugae increases the surface area of the mucosa, aiding in the process of digestion by facilitating the mixing and absorption of nutrients.

The other options pertain to different anatomical structures: the pleura is a membrane surrounding the lungs, the peritoneum is a membrane lining the abdominal cavity, and the diaphragm is a muscle that separates the thoracic cavity from the abdominal cavity. None of these structures relate to the concept of a fold in the mucosal membrane, making "ruga" the correct choice for this description.