**Core Concept**
Metformin, a biguanide, is the primary medication used for the treatment of type 2 diabetes mellitus due to its efficacy in lowering glucose levels by decreasing hepatic glucose production and increasing insulin sensitivity.

**Why the Correct Answer is Right**
Metformin acts by inhibiting mitochondrial respiratory chain complex I, which reduces hepatic glucose production through a reduction in gluconeogenesis. Additionally, metformin increases insulin sensitivity by enhancing the translocation of glucose transporter 4 (GLUT4) to the cell surface. This results in improved glycemic control in patients with type 2 diabetes mellitus.

**Why Each Wrong Option is Incorrect**
**Option A:** Insulin is used for both type 1 and type 2 diabetes mellitus, but it is not the primary medication for type 2 diabetes mellitus. It is often used in combination with metformin or other oral hypoglycemic agents.
**Option B:** Sulfonylureas stimulate insulin release from pancreatic beta cells but are not the primary medication for type 2 diabetes mellitus. They are often used in combination with metformin or other oral hypoglycemic agents.
**Option C:** Thiazolidinediones improve insulin sensitivity but are not the primary medication for type 2 diabetes mellitus. They are often used in combination with metformin or other oral hypoglycemic agents.

**Clinical Pearl / High-Yield Fact**
Metformin is the first-line treatment for type 2 diabetes mellitus due to its efficacy, safety profile, and cost-effectiveness. It is essential to note that metformin is contraindicated in patients with renal impairment, as it can cause lactic acidosis.

**Correct Answer: D. Metformin**