**Core Concept**
Graft rejection is an immune response to a transplanted organ or tissue, where the recipient's immune system recognizes the graft as foreign and mounts an attack against it. This process involves the activation of T cells and the production of antibodies, leading to inflammation and damage to the graft.

**Why the Correct Answer is Right**
The correct answer is based on the concept of immunological tolerance, where the immune system does not recognize the graft as self. Graft rejection occurs when the recipient's immune system fails to achieve tolerance, and instead, recognizes the graft as foreign. This recognition is mediated by T cells that recognize antigens presented by antigen-presenting cells (APCs) in the context of major histocompatibility complex (MHC) molecules. The T cells then activate other immune cells, such as B cells and macrophages, leading to the production of antibodies and the release of inflammatory mediators.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because graft rejection is not a result of the recipient's immune system not recognizing the graft as foreign. Instead, it is the result of the immune system recognizing the graft as foreign and mounting an attack against it.
**Option B:** This option is incorrect because graft rejection is not a result of the recipient's immune system being unable to recognize the graft as self. Instead, it is the result of the immune system recognizing the graft as foreign and mounting an attack against it.
**Option C:** This option is incorrect because graft rejection is not a result of the graft being rejected by the recipient's immune system due to a lack of blood supply. Instead, it is the result of the immune system recognizing the graft as foreign and mounting an attack against it.

**Clinical Pearl / High-Yield Fact**
One key factor that influences the likelihood of graft rejection is the degree of histocompatibility between the donor and recipient. MHC matching between the donor and recipient is critical in reducing the risk of graft rejection.

**Correct Answer: C. Graft rejection is an immune response to a transplanted organ or tissue, where the recipient's immune system recognizes the graft as foreign and mounts an attack against it.**