**Core Concept**
The question is testing the pharmacological principle of immunosuppression, specifically the drugs used to prevent graft rejection in organ transplantation. The correct answer requires knowledge of the immunosuppressive agents that inhibit T-cell activation and proliferation, thereby preventing graft rejection.

**Why the Correct Answer is Right**
Tacrolimus (FK506) is a calcineurin inhibitor that binds to FKBP-12, inhibiting calcineurin's phosphatase activity and thereby blocking the activation of T-cells. This leads to a decrease in the production of interleukin-2 (IL-2), a cytokine essential for T-cell proliferation. By preventing T-cell activation and proliferation, tacrolimus effectively reduces the immune response against the transplanted organ, making it the drug of choice for preventing graft rejection in recipients.

**Why Each Wrong Option is Incorrect**
**Option A:** Corticosteroids, such as prednisone, are used to prevent graft rejection but work by a different mechanism, suppressing the immune response through the inhibition of phospholipase A2 and the subsequent production of inflammatory mediators. They are not the first-line choice for preventing graft rejection.
**Option B:** Cyclosporin A (CsA) is another calcineurin inhibitor, but it is not the most potent or commonly used immunosuppressant for preventing graft rejection. While it does inhibit calcineurin and T-cell activation, tacrolimus is generally more effective and has a faster onset of action.
**Option C:** Sirolimus (rapamycin) is an mTOR inhibitor that suppresses T-cell activation and proliferation by binding to FKBP-12 and inhibiting the mTOR pathway. However, it is not typically used as the primary immunosuppressant for preventing graft rejection.

**Clinical Pearl / High-Yield Fact**
Tacrolimus is known for its narrow therapeutic index, requiring careful monitoring of blood levels to avoid toxicity. Regular monitoring of tacrolimus levels, as well as renal function and electrolytes, is crucial to prevent adverse effects such as nephrotoxicity and neurotoxicity.

**Correct Answer: C. Tacrolimus**