Final answer:
Regulatory T cells (Treg) produce Interleukin-10 (IL-10) to suppress immune responses and maintain immune tolerance, thereby preventing autoimmune diseases.
Step-by-step explanation:
The cytokine produced by regulatory T cells (Treg) to suppress immune responses is Interleukin-10 (IL-10). Regulatory T cells are essential for maintaining immune tolerance and preventing autoimmune diseases by suppressing the activity of other T cells that could otherwise attack self-antigens. Interleukins are cytokines largely produced by immune system cells, and each has its specific functions. IL-10 is known to be a major inhibitor within the immune system, controlling cell-mediated immunity and innate immune reactions. Therefore, the correct answer to the given question is d) Interleukin-10 (IL-10).