The correct option is A Duodenum - pancreas
Secretin is secreted in duodenum and stimulates the secretion of pancreatic juice but inhibits the secretion of gastric juices. It is released in response to acid in the small intestine, and stimulates the pancreas and bile ducts to release a flood of bicarbonate base, which neutralizes the acid.
So, the correct answer is 'Duodenum - pancreas'.