What is the modification of the small intestinal wall that serves to increase surface area

Mucosal fold

Explanation:

The surface area of the small intestinal wall is increased by the modification of the small intestinal wall called Mucosal folds. The interior walls of the small intestine are thrown into closely wrinkled circular projections known as Mucosal folds, which increase the surface area of the small intestine. The Mucosal folds  are made up of mucosal cells which contains microvilli that also increases the surface area.