**Core Concept**
The grading of diabetes mellitus is a clinical classification system used to estimate the severity of the disease based on various factors, including blood glucose levels, symptoms, and complications. This classification system helps in assessing the risk of developing diabetes-related complications and guiding treatment decisions.

**Why the Correct Answer is Right**
The correct classification system used for grading diabetes mellitus is the HbA1c-based classification system. The American Diabetes Association (ADA) recommends using the HbA1c levels to classify diabetes into three categories: mild (HbA1c <7%), moderate (HbA1c 7-9%), and severe (HbA1c ≥9%). This classification system is based on the idea that higher HbA1c levels are associated with increased risk of diabetes-related complications. **Why Each Wrong Option is Incorrect** * **Option A:** A1C is not a classification system, but rather a measure of average blood glucose control over a period of time. * **Option B:** The Apgar score is a classification system used to assess the health of newborn babies, not diabetes. * **Option C:** The Glasgow Coma Scale is a neurological scale used to assess the level of consciousness in patients, not diabetes. **Clinical Pearl / High-Yield Fact** Remember that the HbA1c-based classification system is used to estimate the severity of diabetes, but it is not a substitute for regular clinical evaluation and monitoring of patients. **Correct Answer:** C.