**Core Concept**
The question is testing the understanding of pharmacological interventions that can delay or prevent the onset of type 2 diabetes mellitus in individuals with impaired glucose tolerance. This concept is based on the pathophysiology of type 2 diabetes, where lifestyle interventions and certain medications can influence the progression from impaired glucose tolerance to overt diabetes.

**Why the Correct Answer is Right**
Several interventions have been proven to delay or prevent diabetes in those with impaired glucose tolerance. These include:

* **Metformin**: This biguanide works by decreasing hepatic glucose production, increasing insulin sensitivity, and enhancing glucose uptake in skeletal muscle. Metformin has been shown to reduce the risk of progression to diabetes in individuals with impaired glucose tolerance.
* **Pioglitazone**: This thiazolidinedione increases insulin sensitivity by activating peroxisome proliferator-activated receptor-gamma (PPAR-γ), leading to improved glucose uptake in adipose tissue and skeletal muscle.
* **Lifestyle modifications**: Intensive lifestyle interventions that include diet and exercise have been demonstrated to delay or prevent the onset of diabetes in individuals with impaired glucose tolerance.

**Why Each Wrong Option is Incorrect**
* **Option A:** **No information provided**, but typically, sulfonylureas (e.g., glimepiride) would not be a first-line choice for delaying or preventing diabetes in those with impaired glucose tolerance.
* **Option B:** **No information provided**, but typically, meglitinides (e.g., repaglinide) would not be a first-line choice for delaying or preventing diabetes in those with impaired glucose tolerance.
* **Option C:** **No information provided**, but typically, alpha-glucosidase inhibitors (e.g., acarbose) may have a role in delaying the progression of diabetes, but their effect size is smaller compared to other options.

**Clinical Pearl / High-Yield Fact**
The American Diabetes Association (ADA) recommends metformin as the first-line pharmacologic agent for the prevention of type 2 diabetes in individuals with impaired glucose tolerance.

**Correct Answer: D.**