Final answer:
Lymphoid follicles in the lymph nodes primarily contain B lymphocytes which can differentiate into plasma cells that produce antibodies to fight infections.
Step-by-step explanation:
Lymphoid follicles of the lymph nodes contain a variety of immune cells. The lymphoid follicles of the lymph nodes contain plasma cells.
However, the primary cell type they are known for housing are B lymphocytes, as well as other cells that are a part of the adaptive immune response. When B cells are activated by antigens, they can differentiate into plasma cells. Plasma cells are responsible for producing and secreting antibodies that help in fighting infections. Therefore, the answer to the question is 'a) Plasma cells'.