**Core Concept**
The question is testing the understanding of the stimulatory pathways involved in the activation of naive T cells, a crucial step in the adaptive immune response. The correct answer will require knowledge of the specific ligands and receptors involved in this process.

**Why the Correct Answer is Right**
The most potent stimulator of naive T cells is CD28, a costimulatory receptor expressed on the surface of T cells. When CD28 binds to its ligand, B7-1 (CD80) or B7-2 (CD86), expressed on the surface of antigen-presenting cells (APCs), it provides a crucial signal for the full activation of T cells. This costimulation is essential for the induction of T cell proliferation and differentiation into effector cells. The CD28-B7 interaction also enhances the expression of IL-2 and other cytokines, promoting T cell growth and activation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CD40L (CD154) is a ligand expressed on activated T cells that interacts with CD40 on APCs, but it is not the most potent stimulator of naive T cells.
**Option B:** This option is incorrect because ICOS (Inducible T-cell costimulator) is a costimulatory receptor expressed on T cells that interacts with ICOS-L on APCs, but it is not as potent as CD28 in stimulating naive T cells.
**Option C:** This option is incorrect because OX40 (TNFRSF4) is a costimulatory receptor expressed on activated T cells that interacts with OX40L on APCs, but it is not the most potent stimulator of naive T cells.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that the blockade of CD28-B7 interaction can inhibit T cell activation and has been explored as a therapeutic strategy in autoimmune diseases and transplant rejection.

**Correct Answer:** C. CD28.