**Core Concept**
The **underlying condition** suggested by severe pallor and hepatosplenomegaly in a 6-month-old baby, especially with a similar history in a sibling, points towards a **genetic disorder** affecting the hematopoietic system. This presentation is highly suggestive of a condition that affects the production or function of blood cells.

**Why the Correct Answer is Right**
Given the clinical presentation and familial history, the most likely diagnosis is a **hemoglobinopathy** or a **storage disorder**. However, without the specific answer choices provided, we can infer that the investigation of choice would likely involve **molecular testing** or **histopathological examination** to confirm the diagnosis. This could include tests like **high-performance liquid chromatography (HPLC)** for hemoglobinopathies or **bone marrow biopsy** for other conditions.

**Why Each Wrong Option is Incorrect**
**Option A:** Without knowing the specific test, it's hard to comment, but if it involved a basic test like a complete blood count (CBC), it might not be specific enough for a genetic disorder.
**Option B:** Similarly, without specifics, if this option involved a test that is not directly related to genetic or hematological disorders, it would be incorrect.
**Option C:** If this option suggested an imaging study, while it might show hepatosplenomegaly, it wouldn't provide a specific diagnosis.
**Option D:** If this was a test not commonly used for diagnosing genetic hematological disorders, it would be incorrect.

**Clinical Pearl / High-Yield Fact**
In cases of suspected genetic disorders with hematological manifestations, especially with a family history, **molecular diagnosis** is key. Conditions like **thalassemia** or **sickle cell disease** can present similarly and require specific diagnostic tests for confirmation.

**Correct Answer:** D. Bone Marrow Biopsy