Final answer:
The retroperitoneal organ among the options given is the pancreas.
Step-by-step explanation:
The retroperitoneal organ among the options given is the pancreas. The pancreas is a soft, oblong, glandular organ located transversely in the retroperitoneum, which is the space behind the peritoneum.
It is situated behind the stomach and has both exocrine and endocrine functions. The pancreas secretes digestive enzymes and releases hormones into the blood.