Step 1: A VIPoma is a pancreatic endocrine tumor that secretes large amounts of vasoactive intestinal peptide (VIP). It is also called Verner-Morrison syndrome, pancreatic cholera, or WDHA syndrome.
Step 2: VIP stimulates small intestinal chloride and water secretion. The hallmark is therefore a large-volume secretory diarrhea that persists even during fasting and is almost always more than 1 litre per day, often more than 3 litres per day.
Step 3: This drives the WDHA triad: Watery Diarrhea, Hypokalemia, and Achlorhydria. So the single most classical symptom is the secretory diarrhea, making option b correct.
Step 4: Why not the others: flushing occurs but only in about 20 percent of patients, steatorrhea is usually absent because malabsorption is not the mechanism, and gallstones are linked to somatostatinoma rather than VIPoma.