**Core Concept**
Immunosuppressants are a class of drugs that modulate the immune system to prevent organ rejection in transplant patients or to treat autoimmune diseases. However, some immunosuppressants have also been found to have antitumor properties and are used in the treatment of certain cancers.

**Why the Correct Answer is Right**
The correct answer is **B. Cyclosporine**. Cyclosporine is primarily used as an immunosuppressant to prevent organ rejection in transplant patients. While it has some antiproliferative effects, it is not typically used to treat cancers. In contrast, other options like **A. Sirolimus (Rapamycin)**, **C. Everolimus**, and **D. Tacrolimus** have been studied for their antitumor properties and are used in the treatment of certain cancers, such as renal cell carcinoma and mantle cell lymphoma.

**Why Each Wrong Option is Incorrect**
**Option A:** Sirolimus (Rapamycin) is an mTOR inhibitor that has been shown to have antiproliferative effects and is used to treat certain types of cancer, including renal cell carcinoma and mantle cell lymphoma.
**Option C:** Everolimus is another mTOR inhibitor that is used to treat certain types of cancer, including renal cell carcinoma and subependymal giant cell astrocytoma.
**Option D:** Tacrolimus is a calcineurin inhibitor that has been studied for its antitumor properties and is used in combination with other therapies to treat certain types of cancer, including mantle cell lymphoma.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that immunosuppressants can have both beneficial and detrimental effects on the immune system. While some immunosuppressants may be used to treat cancers, they can also increase the risk of infections and malignancies in transplant patients.

**Correct Answer: B. Cyclosporine, Cyclosporine is primarily used as an immunosuppressant to prevent organ rejection in transplant patients.**