## **Core Concept**
The question tests knowledge of pancreatic islet cell physiology, specifically the secretory products of **beta cells** in the pancreas. Beta cells are crucial for glucose homeostasis through the secretion of **insulin** and another key peptide.

## **Why the Correct Answer is Right**
Beta cells in the pancreatic islets of Langerhans secrete **insulin** in response to elevated blood glucose levels. Along with insulin, beta cells also secrete **C-peptide** (connecting peptide) in equimolar amounts. C-peptide is a byproduct of proinsulin conversion to insulin and serves as a marker for endogenous insulin production.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Glucagon is secreted by **alpha cells** of the pancreas and works opposite to insulin by raising blood glucose levels. Therefore, it is not secreted by beta cells.
- **Option B:** Somatostatin is secreted by **delta cells** of the pancreas and inhibits the secretion of several hormones, including insulin and glucagon. It is not a product of beta cells.
- **Option D:** Pancreatic polypeptide (PP) is secreted by **PP cells** (also known as F cells) and is involved in the regulation of pancreatic exocrine function and glucose metabolism, but not by beta cells.

## **Clinical Pearl / High-Yield Fact**
A key clinical point to remember is that **C-peptide levels** can be measured to assess endogenous insulin production, particularly in differentiating between type 1 diabetes (low C-peptide) and type 2 diabetes or insulin resistance (variable C-peptide levels). This is crucial in managing diabetes and assessing the need for insulin therapy.

## **Correct Answer:** C. C-peptide