The correct option is D All of the above.
There are four major peptide hormones that are secreted by endocrine cells of gastrointestinal tract, these include secretin, gastrin, cholecystokinin (CCK) and gastric inhibitory peptide (GIP).
Secretin acts on the exocrine (glands that release their secretions through a duct) part of the pancreas and stimulates the secretion of water and bicarbonate ions.
Gastrin acts on gastric glands. It is responsible for secretion of hydrochloric acid and pepsinogen.
Cholecystokinin stimulates the secretion of pancreatic enzymes and bile juice by pancreas and gallbladder respectively.