**Core Concept**
Gestational diabetes mellitus (GDM) is a condition characterized by high blood sugar levels first recognized during pregnancy. The glucose tolerance test (GTT) is a diagnostic tool used to assess insulin sensitivity and glucose metabolism in pregnant women.

**Why the Correct Answer is Right**
The correct answer is based on the American Diabetes Association (ADA) criteria for diagnosing GDM. According to the ADA, a 1-hour plasma glucose level of 180 mg/dL or higher during the oral glucose tolerance test (OGTT) is considered abnormal. This threshold is used to identify women who are at risk of developing GDM and require further evaluation and management.

**Why Each Wrong Option is Incorrect**
**Option A:** This value is too low to be considered abnormal. A 1-hour plasma glucose level of 140 mg/dL or less is generally considered normal.
**Option B:** This value is higher than the threshold used by the ADA, but it is not the correct answer.
**Option C:** This value is also higher than the threshold used by the ADA, but it is not the correct answer.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the ADA criteria for diagnosing GDM have changed over time. In 2010, the ADA introduced new criteria that increased the threshold for diagnosing GDM. This change was made to reduce the number of women diagnosed with GDM and to improve the accuracy of the diagnosis.

**Correct Answer:** B. 180 mg/dL.