**Core Concept**
Alpha-glucosidase inhibitors are a class of oral hypoglycemic agents that work by inhibiting the enzyme alpha-glucosidase, which breaks down carbohydrates into simple sugars. This class of drugs is used to treat type 2 diabetes mellitus.

**Why the Correct Answer is Right**
Alpha-glucosidase inhibitors, such as acarbose, work by competitively inhibiting the enzyme alpha-glucosidase in the small intestine. This enzyme is responsible for breaking down disaccharides, such as sucrose and maltose, into simple sugars like glucose and fructose. By inhibiting this enzyme, alpha-glucosidase inhibitors slow down the digestion and absorption of carbohydrates, thereby reducing the postprandial rise in blood glucose levels. This mechanism helps to regulate blood sugar levels in patients with type 2 diabetes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because alpha-glucosidase inhibitors do not stimulate insulin secretion. Instead, they work by slowing down carbohydrate digestion and absorption.
**Option B:** This option is incorrect because metformin is a biguanide class of oral hypoglycemic agents, not an alpha-glucosidase inhibitor. Metformin works by decreasing glucose production in the liver and increasing insulin sensitivity.
**Option C:** This option is incorrect because sulfonylureas stimulate insulin secretion from the pancreas, whereas alpha-glucosidase inhibitors work by slowing down carbohydrate digestion and absorption.

**Clinical Pearl / High-Yield Fact**
Acarbose, a commonly used alpha-glucosidase inhibitor, can cause gastrointestinal side effects like flatulence and diarrhea due to the accumulation of undigested carbohydrates in the colon.

**Correct Answer:** D.