**Core Concept:**
The question is asking about the most suitable diagnostic test for a 5-year-old girl presenting with pallor and hepatosplenomegaly on abdominal examination. Pallor indicates anemia, and hepatosplenomegaly suggests liver or spleen enlargement. These symptoms hint towards a potential disease affecting the blood or hematopoietic system, which might be caused by a congenital or acquired condition.

**Why the Correct Answer is Right:**
The correct answer is **D** - Peripheral blood film examination. A peripheral blood film examination, also known as a blood smear or blood film, is a diagnostic test used to visualize the morphology of red blood cells, white blood cells, and platelets. In this case, it is essential to evaluate the red blood cells (RBCs) for anemia. Anemia can be diagnosed by assessing the size, shape, and color of RBCs. Anemia can be caused by various factors, including congenital disorders, nutritional deficiencies, and hemolytic disorders. The examination can help identify signs of anemia, such as microcytic, hypochromic RBCs, and presence of schistocytes.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Hemoglobin estimation):** Hemoglobin estimation is a test that measures the amount of hemoglobin in the blood. While important for diagnosing anemia, it does not provide information about the morphology of RBCs, which is crucial for diagnosing the underlying cause of anemia.

B. **Option B (Liver function tests):** Liver function tests assess liver function and are not directly related to diagnosing anemia or determining its cause.

C. **Option C (Ultrasound):** Ultrasound is a diagnostic imaging technique used to visualize internal structures and is not a direct test for diagnosing anemia or its underlying cause.

**Clinical Pearl:**
In cases of suspected anemia with hepatosplenomegaly, performing a peripheral blood film examination is essential to evaluate RBC morphology and aid in diagnosing the underlying cause of the anemia. This can help guide further diagnostic procedures or interventions, such as iron studies or a bone marrow examination, depending on the findings of the peripheral blood film.