**Core Concept**
An organ transplant from one individual to another is classified based on the genetic relationship between the donor and recipient. The type of graft is crucial in determining the immune response and potential complications.

**Why the Correct Answer is Right**
An isograft is a type of graft that is transplanted from one individual to another who is genetically identical, such as an identical twin. Since the donor and recipient are genetically identical, there is no risk of graft rejection or immune response. This is because the immune system does not recognize the transplanted tissue as foreign. In this case, the woman receives an organ transplant from her sister who is an identical twin, making it an isograft.

**Why Each Wrong Option is Incorrect**
**Option A:** An allograft is a type of graft that is transplanted from one individual to another who is not genetically identical. This type of graft is more likely to be rejected by the immune system, as the immune system recognizes the transplanted tissue as foreign.

**Option B:** An autograft is a type of graft that is transplanted from one part of the body to another in the same individual. This type of graft is not applicable in this scenario, as the transplant is from one individual to another.

**Option C:** A xenograft is a type of graft that is transplanted from one species to another. This type of graft is not applicable in this scenario, as the transplant is from one human to another.

**Clinical Pearl / High-Yield Fact**
It is essential to note that even though isografts are less likely to be rejected, they can still be affected by other factors such as surgical technique and post-operative care. Identical twins may also have different environmental exposures, which can affect the graft.

**✓ Correct Answer: D. Isograft**