**Core Concept**
The patient's presentation of hypochromic microcytic anemia with a history of multiple blood transfusions is suggestive of iron deficiency anemia. However, given her prolonged exposure to blood transfusions, other potential causes such as hemochromatosis or sideroblastic anemia need to be ruled out. The key here is to consider the differential diagnosis and the relevance of investigations in this context.

**Why the Correct Answer is Right**
The patient's laboratory findings, including low MCV (60 fl), low MCHC (not provided, but implied by hypochromic anemia), and low serum iron, are consistent with iron deficiency anemia. However, given her history of multiple blood transfusions, an investigation that would be crucial in this context is a serum ferritin level, which would help differentiate between iron deficiency anemia and other causes such as hemochromatosis or sideroblastic anemia. Serum ferritin is a key marker of iron stores in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum iron level is not the most critical investigation in this scenario, as it can be influenced by various factors, including inflammation and recent blood transfusions. While it may provide some information, it is not as specific or sensitive as serum ferritin in this context.

**Option B:** Total iron-binding capacity (TIBC) is a useful investigation in iron deficiency anemia, as it can help differentiate between iron deficiency anemia and other causes. However, given the patient's history of multiple blood transfusions, TIBC may not be the most critical investigation.

**Option C:** Hemoglobin electrophoresis is not typically indicated in this scenario, as the patient's presentation is suggestive of iron deficiency anemia rather than a hemoglobinopathy.

**Option D:** Serum ferritin is the most critical investigation in this scenario, as it would help differentiate between iron deficiency anemia and other causes such as hemochromatosis or sideroblastic anemia.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a history of multiple blood transfusions, it is essential to consider the differential diagnosis of iron deficiency anemia, including hemochromatosis and sideroblastic anemia. Serum ferritin is a critical investigation in this context, as it would help differentiate between these conditions.

**Correct Answer:** D. Serum ferritin.