**Core Concept:**
Diabetes mellitus is a group of chronic disorders characterized by high blood glucose levels due to defects in insulin secretion, insulin action, or both. Type 1 diabetes mellitus (DM-1) is an autoimmune disorder where the body's immune system attacks and destroys beta cells in the pancreas, leading to near-total lack of insulin production. In contrast, Type 2 diabetes mellitus (DM-2) is primarily a disorder of insulin resistance, where the body's cells do not respond effectively to insulin, resulting in elevated blood glucose levels.

**Why the Correct Answer is Right:**
The correct answer is C. HbA1c (glycated hemoglobin) is a reliable test for assessing average blood glucose levels over the past 2-3 months. In DM-2, the body has a compensatory hyperinsulinemia, which results in a lower HbA1c value compared to DM-1. In DM-1, there is no insulin production, so HbA1c is usually elevated.

**Why Each Wrong Option is Incorrect:**
A. Fasting plasma glucose (FPG) can be elevated in both DM-1 and DM-2, making it insufficient for differentiation.
B. Oral glucose tolerance test (OGTT) may be abnormal in both types, but HbA1c provides a more sustained picture of average blood glucose levels.
D. Plasma C-peptide levels can be helpful in differentiating the two types, as DM-1 usually has low C-peptide levels, while DM-2 has a compensatory hyperinsulinemia. However, this option is less reliable than HbA1c.

**Clinical Pearl:**
While HbA1c is a highly reliable marker for distinguishing between DM-1 and DM-2, it is essential to consider other clinical and laboratory findings as well, such as age of onset, symptoms, and C-peptide levels. A thorough clinical examination and history are crucial for accurate diagnosis and management of diabetes.