**Core Concept**
The question tests the knowledge of the association between Down syndrome and specific types of leukemia, particularly in children under 3 years old. **Acute leukemias** are the most common type of blood cancer in children with Down syndrome.

**Why the Correct Answer is Right**
In children with Down syndrome, especially those under 3 years old, **Acute Megakaryoblastic Leukemia (AMKL)** is notably prevalent. This is due to the increased susceptibility to hematologic malignancies in Down syndrome patients, with AMKL being a subtype of **Acute Myeloid Leukemia (AML)** that originates from megakaryocyte precursors.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while **Acute Lymphoblastic Leukemia (ALL)** is the most common leukemia in children, it is not the most common in Down syndrome children under 3.
**Option B:** Incorrect as it is not specified, but typically, other forms of leukemia like **Acute Myeloid Leukemia (AML)** are less common in this specific context than AMKL.
**Option D:** Incorrect because, again, it's not specified, but other leukemias are less frequently associated with Down syndrome in this age group compared to AMKL.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that children with Down syndrome have a higher risk of developing leukemia, particularly **AMKL**, which is a significant consideration in their clinical management and follow-up.

**Correct Answer:** D. Acute Megakaryoblastic Leukemia (AMKL)