Final answer:
The unique feature of secondary lymphoid organs is the circulation of lymphocytes; this is where they are activated and mount immune responses. Primary lymphoid organs are where lymphocytes develop and mature, and while cellular proliferation occurs in both primary and secondary organs, only secondary organs are involved in the immune response activation.
Step-by-step explanation:
The unique aspect of secondary lymphoid organs among the options provided is the circulation of lymphocytes. Secondary lymphoid organs, such as lymph nodes, spleen, and lymphoid nodules, are where lymphocytes are activated and mount immune responses after they have matured in the primary lymphoid organs (bone marrow and thymus). While primary lymphoid organs are involved in the maturation and selection of lymphocytes, secondary lymphoid organs are mainly sites for lymphocyte activation and immune responses after lymphocytes encounter antigens.
Presence of precursor of T and B cells typically occurs in primary lymphoid organs, where lymphocyte development begins. The terminal differentiation of lymphocytes also starts in the primary lymphoid organs. Cellular proliferation can occur in both primary and secondary lymphoid organs; however, in the context of secondary lymphoid organs, it is often associated with the clonal expansion of antigen-activated lymphocytes. Answer e, 'All the above', is therefore not correct as the attributes listed do not apply uniquely to secondary lymphoid organs.