**Core Concept**
The question is testing the student's knowledge of selective immunosuppression, specifically the mechanism of action of an agent that targets helper T-cells (Th cells). Helper T-cells play a crucial role in the immune response by activating and directing other immune cells. An agent that selectively inhibits helper T-cells would have a significant impact on the immune response, making it a valuable therapeutic option for autoimmune diseases or organ transplantation.

**Why the Correct Answer is Right**
The correct answer is Cyclosporin A (CsA). CsA acts by inhibiting the activity of calcineurin, a phosphatase that is essential for the activation of helper T-cells. When calcineurin is inhibited, the transcription factor Nuclear Factor of Activated T-cells (NFAT) is unable to enter the nucleus and initiate the transcription of genes involved in T-cell activation. This leads to a selective inhibition of helper T-cell activation, making CsA a powerful immunosuppressive agent. CsA is used to prevent rejection in organ transplantation and to treat autoimmune diseases such as rheumatoid arthritis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Azathioprine is a non-selective immunosuppressive agent that works by inhibiting purine synthesis, which affects both B-cells and T-cells. It is not selective for helper T-cells.
* **Option B:** Tacrolimus (FK506) is another calcineurin inhibitor that affects helper T-cells, but its mechanism of action is slightly different from CsA. It is also not selective for helper T-cells.
* **Option D:** Prednisolone is a corticosteroid that has a broad range of immunosuppressive effects, including the inhibition of helper T-cells. However, its mechanism of action is not selective for helper T-cells.

**Clinical Pearl / High-Yield Fact**
Cyclosporin A (CsA) is a classic example of a selective immunosuppressive agent that has revolutionized the treatment of autoimmune diseases and organ transplantation. Its use has been associated with several side effects, including nephrotoxicity and hypertension, which highlights the importance of careful monitoring and dosing.

**Correct Answer: C. Cyclosporin A (CsA)**