Final answer:
Lymphocytes, including both B cells and T cells, are initially formed in the bone marrow. B cells mature there, while T cells migrate to the thymus for further development.
Step-by-step explanation:
Lymphocytes are a type of white blood cells that play a crucial role in the body's immune system. Lymphocytes are generated from lymphoid stem cells and, although they initially form in the bone marrow, their development continues in other parts of the lymphatic system. There are two main types of lymphocytes: B cells and T cells. B cells complete most of their development in the bone marrow, which is why they are referred to as "B" cells, indicative of their maturation location. T cells, on the other hand, after beginning their development in the bone marrow, migrate to the thymus, where they undergo further maturation; hence the "T" in T cells stands for "thymus". Therefore, since they both originate from bone marrow, the correct answer to the question is: a. Bone marrow.