<prompt> M.C. retroperitoneal tumour is -FibrosarcomaLiposarcomaDermoid cystRhabdosarcoma  <response>: Ans is b ie Liposarcoma The most common primary malignancy of the retroperitoneum is sarcoma.Ref Sabiston In the retroperitoneal location, well-differentiated and dedifferentiated liposarcoma and leiomyosarcoma are the most common histiotypes, whereas in the visceral location, GISTs are found most commonly.- Devitas Oncology 