## Core Concept
The β-thalassemia trait, also known as β-thalassemia minor, is a genetic disorder characterized by reduced production of the β-globin subunit of hemoglobin. It is diagnosed through various laboratory tests that assess hemoglobin levels, red blood cell indices, and genetic analysis.

## Why the Correct Answer is Right
The correct answer, **HbA2 level**, is a key diagnostic criterion for β-thalassemia trait. In β-thalassemia minor, there is a decrease in the production of β-globin chains, which leads to a relative increase in the production of δ-globin chains. As a result, the level of HbA2 (α2δ2) is elevated, typically above 3.5%. This is a specific and sensitive indicator for the diagnosis of β-thalassemia trait.

## Why Each Wrong Option is Incorrect
* **Option A:** While **Complete Blood Count (CBC)** can provide clues suggestive of thalassemia, such as microcytic hypochromic anemia, it is not specific for diagnosing β-thalassemia trait.
* **Option B:** **Peripheral Blood Smear** can show characteristic changes like microcytosis and hypochromia but is not definitive for diagnosing β-thalassemia trait.
* **Option D:** **Bone Marrow Examination** is not a routine test for diagnosing β-thalassemia trait; it may show erythroid hyperplasia but is not specific or sensitive for this condition.

## Clinical Pearl / High-Yield Fact
A crucial clinical pearl is that an elevated **HbA2 level >3.5%** is highly suggestive of β-thalassemia trait. This test is particularly useful in differentiating β-thalassemia minor from other causes of microcytic hypochromic anemia, such as iron deficiency anemia.

## Correct Answer: C. HbA2 level