**Core Concept**
Immunosuppressive agents are a class of medications that prevent or reduce the immune response, often used in organ transplantation and autoimmune diseases. Selective inhibition of helper T-cells, a subset of T lymphocytes, is a specific mechanism of action for some immunosuppressants.

**Why the Correct Answer is Right**
The correct answer acts selectively by inhibiting the activation of helper T-cells (Th cells) through a specific pathway. This agent targets the calcineurin enzyme, which is essential for the transcription of interleukin-2 (IL-2) and other cytokines in T-cells. By inhibiting calcineurin, this immunosuppressant blocks the transcription of IL-2 and other cytokines, leading to a reduction in T-cell proliferation and activation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it targets a different pathway, involving the inhibition of mTOR (mechanistic target of rapamycin) rather than calcineurin. mTOR inhibition affects multiple cell types, including T-cells, but is not selective for helper T-cells.
**Option B:** This option is incorrect because it is a non-selective immunosuppressant that affects both T-cells and B-cells. It does not specifically target helper T-cells.
**Option C:** This option is incorrect because it is a corticosteroid, which has a broad range of immunosuppressive effects, including the inhibition of T-cell activation, but does not specifically target helper T-cells.

**Clinical Pearl / High-Yield Fact**
Remember the "sandwich" effect of calcineurin inhibitors, which inhibit T-cell activation by blocking the transcription of IL-2 and other cytokines, thereby preventing the proliferation and activation of helper T-cells.

**Correct Answer: C. Cyclosporine**