## Core Concept
The question tests knowledge of pancreatic neuroendocrine tumors (PNETs), also known as islet cell tumors. These tumors arise from the endocrine component of the pancreas and can be functioning (secreting hormones) or non-functioning. The most common functional PNETs secrete various hormones, leading to distinct clinical syndromes.

## Why the Correct Answer is Right
The correct answer, **Insulinoma**, is the most common functional neuroendocrine tumor of the pancreas. Insulinomas secrete insulin, leading to hypoglycemia. They are typically small, solitary, and benign. The symptoms of insulinoma include episodes of hypoglycemia, which can manifest as confusion, shakiness, dizziness, hunger, headaches, sweating, and palpitations.

## Why Each Wrong Option is Incorrect
* **Option A: Glucagonoma** - This tumor secretes glucagon, leading to a syndrome characterized by diabetes mellitus, weight loss, and a distinctive rash (necrolytic migratory erythema). While it is a functional PNET, it is less common than insulinoma.
* **Option B: Gastrinoma** - This tumor secretes gastrin, causing excessive acid production in the stomach, leading to peptic ulcers (Zollinger-Ellison syndrome). It is another type of functional PNET but is less common than insulinoma.
* **Option D: Somatostatinoma** - This rare tumor secretes somatostatin, leading to diabetes mellitus, gallstones, and malabsorption. It is much less common than insulinoma.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that **insulinomas are often small and can be difficult to localize**, but they are usually benign and solitary. A diagnostic clue is that symptoms often improve with glucose administration. Preoperative localization of the tumor can be challenging, but techniques like CT, MRI, and endoscopic ultrasound are commonly used.

## Correct Answer: B. Insulinoma