**Core Concept:** Naïve T cells are a type of white blood cell that plays a crucial role in adaptive immune response, specifically in cell-mediated immunity. They are activated through a process involving T cell receptor (TCR) engagement with antigen-presenting cells (APCs) and co-stimulatory signals.

**Why the Correct Answer is Right:** Interleukin-2 (IL-2) is the most potent stimulator of naïve T cells. IL-2 is a cytokine produced by activated T cells and natural killer (NK) cells, which binds to the high-affinity IL-2 receptor (IL-2R) on the surface of naïve T cells. This binding activates the cells, leading to their proliferation and differentiation into effector T cells.

**Why Each Wrong Option is Incorrect:**

A. Interleukin-4 (IL-4) is a cytokine involved in the differentiation of naïve B cells into plasma cells, not naïve T cells.

B. Interleukin-7 (IL-7) is primarily involved in the growth and survival of thymocytes, immature T cells in the thymus, rather than naïve T cells.

C. Interleukin-15 (IL-15) is involved in the activation and survival of memory T cells, not naïve T cells.

D. Interleukin-12 (IL-12) is involved in the activation and differentiation of natural killer cells (NK) and macrophages, not naïve T cells.

**Clinical Pearl:** The understanding of cytokines and their roles in immune system activation is essential for understanding immunology and treating immune-mediated diseases.

**Correct Answer:** D. Interleukin-15 (IL-15) is the correct answer because it is the most potent stimulator of naïve T cells. IL-15 acts on naïve T cells to promote their activation and survival, enabling them to differentiate into effector and memory T cells. Understanding the role of cytokines like IL-15 in immune system activation can help in treating immune-mediated diseases and assessing the effectiveness of immune therapy.