**Core Concept**
The long-term survival of an allograft depends on the ability of the recipient's immune system to tolerate the graft without initiating an immune response. This is achieved through various mechanisms, including immunosuppression, antigen presentation, and tolerance induction.

**Why the Correct Answer is Right**
The correct answer is based on the concept of mixed chimerism, where the recipient's immune system is exposed to a small amount of the donor's immune cells, leading to a state of tolerance. This is achieved through the use of hematopoietic stem cell transplantation, which allows for the coexistence of the recipient's and donor's immune cells. The recipient's immune system learns to accept the donor's cells as "self," reducing the risk of graft rejection and increasing the chances of long-term survival.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the underlying mechanism of long-term survival of an allograft. While immunosuppressive drugs can help prevent rejection, they do not induce tolerance and do not provide long-term protection.

**Option B:** This option is incorrect because it is not a recognized method for achieving long-term survival of an allograft. Bone marrow transplantation can be used to treat certain diseases, but it is not a reliable method for achieving tolerance to an allograft.

**Option C:** This option is incorrect because it is not a specific enough answer. While it is true that reducing the intensity of immunosuppression can lead to better graft survival, it is not a specific enough answer to be the correct choice.

**Clinical Pearl / High-Yield Fact**
Mixed chimerism is a state of tolerance where the recipient's immune system coexists with the donor's immune cells, reducing the risk of graft rejection and increasing the chances of long-term survival.

**Correct Answer:** B. Bone marrow transplantation with mixed chimerism is a promising approach for achieving long-term survival of an allograft.