A) This statement is correct. Having a family history of leukemia is considered a risk factor for developing the disease, particularly in cases of familial or hereditary leukemia.

B) This statement is correct. Exposure to high levels of ionizing radiation, such as during radiation therapy or nuclear accidents, is a well-known risk factor for leukemia.

C) This statement is correct. Individuals with Down syndrome have an increased risk of developing leukemia, particularly acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML).

D) This statement is incorrect. While physical activity is beneficial for overall health and may reduce the risk of various cancers, there is no evidence to suggest that being physically active specifically reduces the risk of developing leukemia.