duodenum
This question is asking you to identify the organ that is secondarily retroperitoneal. This means that it started out peritonealized but became pressed against the posterior body wall and stuck there during development. Except for the first few centimeters of the superior segment and the duodenojejunal junction, the duodenum is a secondarily retroperitoneal organ - it used to have a mesentery, but that was lost during gut rotation. Although the kidney is a retroperitoneal organ, it is not secondarily retroperitoneal - it started developing in the retroperitoneum and stayed there. The spleen, stomach, and transverse colon are all peritonealized. What segments of the colon are peritonealized? The cecum, transverse colon, and the sigmoid colon are peritoneal, but the ascending and descending colon are retroperitoneal.