**Core Concept**
The diagnosis of acute leukemia is primarily based on the percentage of blast cells in the peripheral blood or bone marrow. Blast cells are immature white blood cells that are not yet fully developed. The World Health Organization (WHO) criteria for the diagnosis of acute leukemia require the presence of at least 20% blast cells in the bone marrow or 30% in the blood.

**Why the Correct Answer is Right**
The correct answer is B. 20 because the WHO criteria for the diagnosis of acute leukemia specify that at least 20% blast cells must be present in the bone marrow or 30% in the peripheral blood. This threshold helps to distinguish acute leukemia from other bone marrow disorders, such as myelodysplastic syndromes or myeloproliferative neoplasms. The presence of blast cells in the bone marrow or blood indicates a clonal proliferation of malignant hematopoietic cells.

**Why Each Wrong Option is Incorrect**
**Option A:** 10% is too low to meet the WHO criteria for the diagnosis of acute leukemia.
**Option C:** 30% in the bone marrow is not a valid criterion for diagnosing acute leukemia; the correct criterion is 20% in the bone marrow or 30% in the blood.
**Option D:** 40% is a higher percentage than required by the WHO criteria, which makes it an incorrect option.

**Clinical Pearl / High-Yield Fact**
To remember the correct percentage, recall the WHO criteria: at least 20% blast cells in the bone marrow or 30% in the blood. This will help you to distinguish acute leukemia from other bone marrow disorders.

**✓ Correct Answer: B. 20**