**Core Concept**
The diagnosis of diabetes mellitus involves measuring blood glucose levels, specifically the fasting blood glucose (FBG) level, which is an essential criterion for diagnosing diabetes. The American Diabetes Association (ADA) and the World Health Organization (WHO) provide guidelines for diagnosing diabetes based on FBG levels.

**Why the Correct Answer is Right**
The correct answer is **A. 126 mg/dl** because, according to the ADA and WHO guidelines, a fasting blood glucose level of 126 mg/dl or higher is diagnostic of diabetes mellitus. This is because a level of 126 mg/dl or higher indicates impaired fasting glucose regulation, which is a hallmark of diabetes. The ADA and WHO recommend using a glucose meter or laboratory testing to measure FBG levels, and a level of 126 mg/dl or higher is considered abnormal.

**Why Each Wrong Option is Incorrect**
**Option B:** 140 mg/dt is incorrect because it is not a valid unit of measurement for glucose levels. Glucose levels are typically measured in milligrams per deciliter (mg/dl) or millimoles per liter (mmol/L), not milligrams per deciliter (mg/dt).

**Option C:** 100 mg/dl is incorrect because this level is actually considered normal, not diagnostic of diabetes. A fasting blood glucose level of 100 mg/dl or lower is within the normal range.

**Option D:** 200 mg/dl is incorrect because, while it is indeed a high glucose level, it is not the specific threshold used for diagnosing diabetes. The ADA and WHO guidelines recommend a level of 126 mg/dl or higher for diagnosis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that impaired fasting glucose (IFG) is defined as a fasting blood glucose level between 100 mg/dl and 125 mg/dl. IFG is a pre-diabetic state that increases the risk of developing type 2 diabetes.

**✓ Correct Answer: A. 126 mg/dl**