Final answer:
The correct answer to the question is A. BAFF, which stands for B-cell activating factor and is essential for the survival and function of B cells in the secondary lymphoid tissues.
Step-by-step explanation:
The critical survival cytokine for B cells in the secondary lymphoid tissue cortex is BAFF (B-cell activating factor). B cells rely heavily on BAFF for their survival, proliferation, and differentiation in the follicular areas of the secondary lymphoid tissues, such as the spleen and lymph nodes. BAFF is not to be confused with interleukins, which are a different group of cytokines. Interleukins primarily associated with the production of various lymphocytes and immune responses. T cells secrete different cytokines like interleukins and interferons, which help in the proliferation and maturation of B cells into antibody-producing plasma cells.