Select the form of leukemia that affects the bone marrow and develops gradually. A. Acute myelocytic leukemia C. Acute lymphocytic leukemia B. Chronic myelocytic leukemia D. Chronic lymphocytic leukemia

To determine the form of leukemia that affects the bone marrow and develops gradually, we need to identify the characteristics of the different types of leukemia provided as options. Let's break down each option.

A. Acute myelocytic leukemia (AML) is a type of leukemia that develops rapidly, affecting the myeloid cells in the bone marrow. It does not fit the description of developing gradually.

B. Chronic myelocytic leukemia (CML) is a type of leukemia that develops slowly, initially remaining in the chronic phase. However, CML typically does not affect the bone marrow as the question specifies.

C. Acute lymphocytic leukemia (ALL) is a type of leukemia that usually affects children and develops rapidly. It also affects the lymphoid cells rather than the bone marrow.

D. Chronic lymphocytic leukemia (CLL) is a form of leukemia that usually progresses slowly, often over many years. It predominantly affects the lymphoid cells but can also influence the bone marrow, making it the most appropriate option for the given description.

Therefore, the correct answer is D. Chronic lymphocytic leukemia (CLL).