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: Medicine
Which of the following is the most common functioning pancreatic islet cell tumour ? 
Select and explain. 
A. Gastrinoma, B. Glucagonoma, C. Insulinoma, D. Vipoma
C: Insulinoma
Answer: C: Insulinoma 
Explanation: None