**Core Concept**
The question is testing the student's knowledge of the pharmacological properties of antidiabetic drugs, specifically their effects on body weight. Type-II diabetes mellitus is a metabolic disorder characterized by insulin resistance and impaired insulin secretion, often associated with obesity.

**Why the Correct Answer is Right**
Metformin is a biguanide antidiabetic drug that works by decreasing hepatic glucose production and increasing insulin sensitivity. One of its notable side effects is weight loss, which is thought to be due to the reduction in hepatic glucose output and increased satiety. Additionally, metformin may also increase the expression of genes involved in fat oxidation, leading to weight loss.

**Why Each Wrong Option is Incorrect**
**Option A:** Sulfonylureas, such as glipizide, actually cause weight gain due to their hypoglycemic effect, which stimulates appetite and increases food intake.
**Option B:** Thiazolidinediones, like pioglitazone, may cause weight gain due to fluid retention and increased fat deposition.
**Option C:** DPP-4 inhibitors, such as sitagliptin, are weight-neutral and do not have a significant effect on body weight.

**Clinical Pearl / High-Yield Fact**
Metformin is often considered a first-line treatment for type-II diabetes due to its efficacy in lowering HbA1c and its potential to cause weight loss, which is beneficial for patients with insulin resistance.

**Correct Answer:** B. Pioglitazone