**Core Concept**
The American College of Obstetricians and Gynecologists (ACOG) 2001 criteria for diagnosing gestational diabetes mellitus (GDM) using the glucose tolerance test (GTT) involves measuring plasma glucose levels at specific time points. This criteria helps identify women at risk of developing GDM during pregnancy.

**Why the Correct Answer is Right**
According to the ACOG 2001 criteria, the diagnosis of GDM is made if any two or more of the following plasma glucose values are met or exceeded. For the 2-hour post-glucose challenge test, the threshold is set at 199 mg/dL. This value is crucial for identifying women with impaired glucose tolerance, who are at risk of developing GDM and its associated complications.

**Why Each Wrong Option is Incorrect**
* **Option A:** 140 mg/dL is too low to be the threshold for diagnosing GDM. This value is closer to the normal range for a 2-hour post-glucose challenge test.
* **Option B:** 220 mg/dL is too high to be the threshold for diagnosing GDM. This value is above the typical range for a 2-hour post-glucose challenge test.

**Clinical Pearl / High-Yield Fact**
The ACOG 2001 criteria for diagnosing GDM using the GTT has been widely adopted, but it has been replaced by the Carpenter and Coustan criteria in some guidelines. It's essential for healthcare providers to be familiar with the specific criteria used in their institution or region.

**Correct Answer: C. 199 mg/dL.**