**Core Concept**
The question is based on the diagnosis of anemia, specifically focusing on the differentiation between iron deficiency anemia and thalassemia. The **Mentzer index**, which is calculated by dividing the mean corpuscular volume (**MCV**) by the red blood cell count (**RBC**), is used to distinguish between these two conditions.

**Why the Correct Answer is Right**
Given the **Mentzer index** of 12, which is less than 13, it suggests thalassemia. The patient's **MCV** of 55 fl indicates microcytic anemia, and the **RBC** count is elevated, which is a characteristic feature of thalassemia trait. The absence of a history of blood transfusion also supports this diagnosis, as patients with thalassemia major often require frequent blood transfusions.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect because a Mentzer index above 13 typically points towards iron deficiency anemia.
**Option B:** and **Option C:** Are not provided, but typically, other options might include other types of anemia or incorrect calculations.

**Clinical Pearl / High-Yield Fact**
The **Mentzer index** is a useful tool in differentiating between iron deficiency anemia and thalassemia, with values below 13 suggesting thalassemia and values above 13 indicating iron deficiency anemia. Remembering this index can help in quickly identifying the cause of microcytic anemia.

**Correct Answer:** D. Thalassemia Trait.