**Core Concept**
Fasting plasma glucose (FPG) or glycated hemoglobin (HbA1c) levels are used for the screening and diagnosis of diabetes mellitus. The most reliable test is crucial for early detection and management of the disease.

**Why the Correct Answer is Right**
The most reliable test for screening diabetes mellitus is the HbA1c test. This is because HbA1c measures the average blood glucose levels over the past 2-3 months, providing a more accurate reflection of long-term glucose control. The HbA1c test is also more convenient and less prone to variability compared to fasting plasma glucose (FPG) tests. In addition, HbA1c levels are not affected by the time of day or recent meals, making it a more reliable option for screening.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fasting plasma glucose (FPG) tests measure glucose levels at a single point in time and may not accurately reflect long-term glucose control. FPG tests are also more prone to variability and may be affected by recent meals and the time of day.
* **Option B:** Oral glucose tolerance test (OGTT) measures glucose levels after consuming a sugary drink. While OGTT is useful for diagnosing gestational diabetes and impaired glucose tolerance, it is not the most reliable test for screening diabetes mellitus in general populations.
* **Option C:** Random plasma glucose (RPG) tests measure glucose levels at any time without regard to food intake. RPG tests are not reliable for screening diabetes mellitus due to the variability in glucose levels throughout the day.

**Clinical Pearl / High-Yield Fact**
The American Diabetes Association recommends using HbA1c levels ≥ 6.5% or FPG levels ≥ 126 mg/dL for the diagnosis of diabetes mellitus. Early detection and management of diabetes mellitus are crucial for preventing complications such as cardiovascular disease, nephropathy, and retinopathy.

**Correct Answer: D. HbA1c test**