**Core Concept:** Insulin is a peptide hormone secreted by beta cells of the pancreas, which plays a crucial role in regulating glucose homeostasis in the body. Insulin secretion is stimulated by elevated blood glucose levels and inhibited by low glucose levels.

**Why the Correct Answer is Right:** Insulin secretion is primarily regulated by the hormone glucose-dependent insulinotropic polypeptide (GIP), glucagon, and the incretin effect. The incretin effect refers to the augmentation of insulin secretion by nutrients entering the circulation, like glucose. When blood glucose levels rise, the pancreas releases GIP and incretin hormones, which stimulate insulin secretion.

**Why Each Wrong Option is Incorrect:**

A. Glucagon is a hormone released from the alpha cells of the pancreas, and while it does stimulate glucose-induced insulin secretion, it is not the correct answer because glucose-dependent insulinotropic polypeptide (GIP) and incretin effect play a more dominant role in insulin secretion.

B. Insulin secretion is not directly stimulated by glucagon. While it may indirectly contribute to insulin secretion, the primary stimulus for insulin release is glucose-dependent insulinotropic polypeptide (GIP) and incretin effect.

C. Insulin secretion is not stimulated by leptin, a hormone produced by adipose tissue, as its primary function is to regulate energy homeostasis, not glucose homeostasis.

D. Somatostatin is a hormone produced by the pancreas and gastrointestinal tract, which inhibits insulin secretion and other hormone release. However, it is not the correct answer because glucose-dependent insulinotropic polypeptide (GIP) and incretin effect play a more significant role in insulin secretion.

**Clinical Pearl:**

Understanding the complex interplay between various hormones like GIP, incretin effect, glucagon, leptin, and somatostatin is essential for understanding glucose homeostasis and the regulation of insulin secretion. The correct answer is GIP and the incretin effect, which play a dominant role in stimulating insulin secretion in response to elevated blood glucose levels.