Final answer:
The correct answer is a) Bone marrow, which is considered a primary lymphoid organ where B cells mature and all lymphocytes are initially produced.
Step-by-step explanation:
The primary lymphoid organ mentioned in the options provided is a) Bone marrow. Primary lymphoid organs are where lymphocytes, such as B cells and T cells, are produced and undergo maturation. The bone marrow is the site where all lymphocytes are produced initially. B cells mature here, and while T cells are also produced in the bone marrow, they travel to the thymus to mature, which is also considered a primary lymphoid organ. In contrast, secondary lymphoid organs, such as lymph nodes, spleen, and tonsils, are involved in filtering lymph and storing lymphocytes; they are sites where pathogens encounter and activate lymphocytes, which initiates adaptive immune responses.
Therefore, the correct answer is "a) Bone marrow".