## Core Concept
The question tests the understanding of glucose-mediated insulin secretion, a fundamental process in endocrinology. **Glucose-stimulated insulin secretion** is a critical mechanism by which the body maintains glucose homeostasis. This process involves the uptake of glucose by pancreatic beta cells, leading to an increase in intracellular ATP, which in turn closes potassium channels and depolarizes the cell.

## Why the Correct Answer is Right
The correct answer, **Closure of potassium channels**, is right because when glucose enters pancreatic beta cells, it is metabolized to produce ATP. The increase in the ATP/ADP ratio leads to the **closure of ATP-sensitive potassium channels**. This closure prevents potassium ions from leaving the cell, causing **depolarization** of the beta cell membrane. Depolarization opens voltage-gated calcium channels, allowing an influx of calcium ions into the cell. The increase in intracellular calcium triggers the exocytosis of insulin-containing vesicles, leading to an increase in plasma insulin levels.

## Why Each Wrong Option is Incorrect
- **Option A:** Increased glucagon-like peptide-1 (GLP-1) secretion is an incretin effect that does enhance insulin secretion but is not the direct mechanism by which glucose increases plasma insulin. GLP-1 acts through specific receptors on beta cells to augment glucose-stimulated insulin secretion.
- **Option B:** Activation of protein kinase C (PKC) is involved in various signaling pathways, including those in pancreatic beta cells. However, the primary and direct mechanism of glucose-stimulated insulin secretion involves changes in ATP/ADP ratios and potassium channel activity, not the activation of PKC.
- **Option D:** Decreased glucagon secretion is a consequence of elevated glucose and insulin levels but is not the mechanism by which glucose directly increases plasma insulin.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **sulfonylureas**, a class of drugs used to treat type 2 diabetes, work by **closing ATP-sensitive potassium channels** in pancreatic beta cells, mimicking the effect of increased glucose and thereby stimulating insulin release. This highlights the importance of potassium channels in the regulation of insulin secretion.

## Correct Answer: C. Closure of potassium channels.