**Core Concept**
Graft-versus-host disease (GVHD) is a major complication of bone marrow transplantation, occurring when donor T-cells recognize the recipient's tissues as foreign and mount an immune response against them. The prevention of GVHD is crucial in ensuring the success of bone marrow transplantation.

**Why the Correct Answer is Right**
The correct method of prevention of GVHD is the use of **immunosuppressive medications**, specifically **cyclosporine**. Cyclosporine works by inhibiting the activity of T-cells, thereby reducing the risk of GVHD. It acts by binding to cyclophilin, which then inhibits calcineurin, a key enzyme involved in the activation of T-cells. This results in a decrease in the production of interleukin-2 (IL-2) and other cytokines, leading to a suppression of the immune response and a reduced risk of GVHD.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not specified, but if it were a different immunosuppressive medication, it would not be the correct answer as cyclosporine is the most commonly used agent for GVHD prevention.
* **Option B:** The use of **total body irradiation (TBI)** is actually a treatment for GVHD, not a method of prevention. TBI is used to deplete the donor T-cells and prevent GVHD.
* **Option C:** **Bone marrow ablation** is not a method of preventing GVHD, but rather a treatment for certain types of cancer. It involves the use of high-dose chemotherapy and/or radiation to destroy the bone marrow and prevent the growth of cancer cells.

**Clinical Pearl / High-Yield Fact**
The use of cyclosporine for GVHD prevention is a classic example of the importance of immunosuppression in preventing graft rejection and GVHD. It is essential to remember that GVHD is a major complication of bone marrow transplantation and that immunosuppressive medications are a crucial part of preventing it.

**Correct Answer:** C. Cyclosporine