The correct option is D Both a and b
Humans have a pair of kidneys for excretion purposes. Although a single kidney is sufficient for excretion, humans have the other kidney as backup (in case one kidney fails). Hence anyone (with a healthy kidney) can donate one kidney and survive with a single kidney.
For a successful kidney transplantation, the kidney from the donor should match with the recipient. Otherwise there could be a chance of kidney rejection from the immune system. Thus the best match for kidney transplantation will be identical twins because identical twins are genetically similar in every aspect. In case a person does not have identical twin, then close relatives would be the best match.