Primary lymphoid organs : Immature lymphocytes differentiates into antigen sensitive lymphocyte :: Secondary Lymphoid organs : _____________________________.

The correct option is B Site for interaction of lymphocytes with antigen
Lymphoid organs are the organs where origin and/or maturation and proliferation of lymphocytes occur.

Primary lymphoid organs are the organs where both origin (bone marrow), maturation and proliferation of lymphocytes occur. Examples are bone marrow (origin and maturation of B-lymphocytes) and thymus (maturation of T-lymphocytes).

Secondary lymphoid organs provide sites for interaction of T-lymphocytes and B-lymphocytes with antigens then they turn into effector cells. Effector cells are short-lived cells actively responds to a stimulus or foreign substance. Hence option b is correct and option d is incorrect.

Figure : Function of lymphoid organ

Lymphatic sytem with lymph vessels helps in the tissue fluid movement. Interaction of RBCs and WBCs is not the function of secondary lymphoid organs. Hence option a and c are incorrect.