Final answer:
Option (a), T helper 2 cells secrete cytokines such as IL-5, which is important for the growth and differentiation of B cells and the activation of eosinophils.
Step-by-step explanation:
The cytokines secreted by T helper 2 (Th2) cells include IL-4, IL-5, IL-6, IL-10, and IL-13. Th2 cells are involved in the humoral immune response, which is the aspect of immunity that is mediated by secreted antibodies produced by B cells.
Among the options provided, IL-5 is a cytokine that is characteristically secreted by T helper 2 cells and is vital in promoting the growth and differentiation of B cells into plasma cells, as well as eosinophil activation, which is important in combating parasitic infections.