**Core Concept**
Transplantation involves the transfer of a graft (organ, tissue, or cell) from a donor site to a recipient site. The type of graft is classified based on the genetic similarity between the donor and recipient. Autografts, allografts, isografts, and xenografts are the main categories, each with distinct characteristics.

**Why the Correct Answer is Right**
In the context of renal transplantation, an allograft is a graft that is transplanted between two genetically different individuals of the same species. In this case, the mother acts as a donor of one kidney to her son, making them genetically different but of the same species (human). This is an example of an allograft, as it involves the transfer of a kidney from a genetically non-identical donor to a recipient.

**Why Each Wrong Option is Incorrect**
**Option A:** Autograft refers to a graft that is transplanted from one part of the body to another within the same individual. This does not apply to the scenario of a mother donating a kidney to her son.

**Option B:** Heterograft is an outdated term that was previously used to describe an allograft. It is not the correct term to use in modern medical literature.

**Option D:** Xenograft refers to a graft that is transplanted from a donor of a different species to a recipient of the same species. This would involve transplanting a kidney from an animal to a human, which is not the case in this scenario.

**Clinical Pearl / High-Yield Fact**
When considering organ donation, it is essential to understand the different types of grafts and their implications for transplantation. Allografts are the most common type of graft and have become increasingly successful with advancements in immunosuppressive therapy.

✓ Correct Answer: C. Allograft