**Core Concept**
Diabetes mellitus is a group of metabolic disorders characterized by hyperglycemia (high blood sugar levels), which can be diagnosed using various laboratory tests. These tests measure different aspects of glucose metabolism, insulin function, and pancreatic beta-cell function.

**Why the Correct Answer is Right**
The diagnosis of diabetes mellitus involves assessing fasting glucose levels, postprandial glucose levels, and hemoglobin A1c (HbA1c). Fasting plasma glucose (FPG) measures glucose levels after an overnight fast, while postprandial glucose measures glucose levels 2 hours after a meal. HbA1c reflects average blood glucose levels over the past 2-3 months. These tests provide valuable information about glucose metabolism and help diagnose diabetes mellitus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a standard test for diagnosing diabetes mellitus. The correct options for diagnosing diabetes mellitus include fasting plasma glucose, postprandial glucose, and hemoglobin A1c.

**Option B:** This option is incorrect because it is not a specific test for diabetes mellitus. However, it may be used in the context of diabetes management or as an indicator of glucose metabolism.

**Option C:** This option is incorrect because it is not a primary test for diagnosing diabetes mellitus. While it may be used in the context of diabetes management, it is not a standard diagnostic tool.

**Clinical Pearl / High-Yield Fact**
The American Diabetes Association recommends using a combination of fasting plasma glucose, postprandial glucose, or hemoglobin A1c for diagnosing diabetes mellitus. It is essential to note that these tests may be used in conjunction with clinical evaluation and patient history to make an accurate diagnosis.

**Correct Answer: D.**