The safest option is (2) Transforming skin cells from patient into iPS cells and using them for further differentiation and grafting in liver.

Option-wise explanation

(1) Transplanting adult liver cells from a healthy donor

• Uses allogeneic hepatocytes, which can provide functional liver tissue but carry a high risk of immune rejection, requiring lifelong immunosuppression.

• Also limited by donor availability.

• Safer than embryonic stem cells, but not the safest among the options.

(2) Patient skin → iPS cells → liver graft – Safest

• Induced pluripotent stem (iPS) cells generated from the patient’s own somatic cells (e.g., skin fibroblasts) are autologous.

• After controlled differentiation into hepatocyte-like cells, grafting back minimizes immune rejection and avoids ethical issues tied to embryos.

• Tumor risk is lower than injecting undifferentiated pluripotent cells, because differentiated cells are transplanted rather than raw iPS cells.

(3) Injecting embryonic stem cells into damaged liver

• Embryonic stem cells are highly pluripotent but allogeneic and prone to forming teratomas/tumors if not fully differentiated.

• Direct injection into liver is considered unsafe due to uncontrolled differentiation and strong ethical/regulatory concerns.

(4) Injecting cord blood cells directly into liver

• Cord blood contains mainly hematopoietic stem cells, not liver‑committed progenitors.

• Their ability to engraft and regenerate liver tissue is limited; direct injection is unlikely to be effective or well‑controlled, and may still cause immune issues if not HLA‑matched.

Given these considerations, the most rational and safest therapeutic strategy for liver regeneration is patient‑specific iPS cell–based therapy (option 2), where cells are autologous and pre‑differentiated toward the hepatic lineage before transplantation.