In which organ of the human body are the lymphocyte cells formed?

Lymphocytes are produced from hematopoietic stem cells in the bone marrow, which is a primary (central) lymphoid tissue where B cells develop and all lymphocytes originate; T‑cell precursors produced in marrow migrate to the thymus for maturation [1]. The red bone marrow within long bones is therefore the principal site of lymphopoiesis in adults and children, and is widely described as the source of both B and precursor T lymphocytes. Secondary lymphoid organs such as the spleen serve mainly for lymphocyte activation and antigen trapping, not primary lymphocyte formation. Therefore, among the options given, the long bone (bone marrow) is the correct site of lymphocyte formation.

Sources

1. [1] https://www.sciencedirect.com/topics/neuroscience/lymphatic-tissue