which of the following is not a secondary lymphoid organ

1. lymph nodes & bone marrow
2. lymph node & thymus
3. bone marrow & thymus
4. lymph node, bone marrow & thymus

Hello Student
Correct answer is 3) bone marrow and thymus
Bone marrowis the main lymphoid organ where all blood cell including lymphocytes are produced.
Thymusis a bilobed organ located near the heart, beneath the breastbone.
Thymus and bone marrow are primary lymphoid organ where T and B lymphocytes mature lymphoid

The correct answer is (3) bone marrow and thymus.
The thymus and the bone marrow constitute the primary lymphoid organs involved in the production and early clonal selection of lymphocyte tissues. Bone marrow is responsible for the creation of both T-cells, and the production and maturation of B-cells. From the bone marrow B-cells immediately join the blood system and travel to secondary lymphoid organs in search of pathogens. T-cells on the other hand travel from the bone marrow to the thymus where they are allowed to develop further. Mature T-cells join B-cells in search of pathogens.

Lymph nodes are secondary lyphoid organs.