This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
The most important action of Secretin is to 
Select and explain. 
A. neutralize the acid from the stomach, B. increase secretion of bicarbonates by pancreas, C. decrease gastric secretion, D. cause contraction of pyloric sphincter
B: increase secretion of bicarbonates by pancreas
Answer: B: increase secretion of bicarbonates by pancreas 
Explanation: Main Action of Secretin:
Secretin increases the secretion of bicarbonate by the duct cells of the pancreas and biliary tract. It thus causes the secretion of a watery, alkaline pancreatic juice (with high content of bicarbonate). Its action on pancreatic duct cells is mediated via cAMP.
Secretin causes alkaline pancreatic juice to flood into the duodenum, neutralizing the acid from the stomach and thus inhibiting further secretion of the hormone - feedback control.
Other actions:

Inhibits gastric juice secretion.
Inhibits gastric motility.
Causes contraction of pyloric sphincter.
Augments the potency of action of cholecystokinin on pancreatic secretion.