For which one of the following tumors, Gastrin is a biochemical marker -
Gastrin is secreted by Gastrinomas.
Gastrinomas are neuroendocrine tumors found in


Duodenum (50 to 70%)
Pancreas (20 to 40%) and 
Other intraabdominal sites (mesentry, lymph nodes, biliary tract, liver; stomach, ovary).
Medullary carcinoma of thyroid
Pancreatic neuroendocrine tumor
Pheochromocytoma
Gastrointestinal stromal tumor