**Core Concept**
Tissue graft rejection occurs when the immune system recognizes the transplanted tissue as foreign and mounts an immune response against it. To prevent rejection, immunosuppressive therapy is used to suppress the immune response.

**Why the Correct Answer is Right**
The correct answer is related to the suppression of T-cell activation, which plays a crucial role in the immune response against transplanted tissues. Cyclosporine works by inhibiting the transcription of cytokines, such as IL-2, which are essential for T-cell activation and proliferation. By suppressing T-cell activation, cyclosporine reduces the immune response against the transplanted tissue, thereby preventing rejection.

**Why Each Wrong Option is Incorrect**
**Option A:** Corticosteroids, while effective in reducing inflammation, do not specifically target T-cell activation and have a broader range of immunosuppressive effects.

**Option B:** Tacrolimus, while similar to cyclosporine in mechanism, is not the primary choice for preventing tissue graft rejection.

**Option C:** Sirolimus, also known as rapamycin, inhibits T-cell activation but through a different mechanism than cyclosporine, involving inhibition of the mTOR pathway.

**Clinical Pearl / High-Yield Fact**
Cyclosporine is a calcineurin inhibitor that suppresses T-cell activation by inhibiting the transcription of cytokines, making it a cornerstone in the prevention of tissue graft rejection.

**Correct Answer:** A. Cyclosporine