Final answer:
The kidney is the most frequently donated and transplanted organ due to the possibility of living donations and the adaptability of the kidney to transplantation procedures.
Step-by-step explanation:
The organ most frequently donated and transplanted is the kidney. Compared to other vital organs like the heart and liver, kidneys are more commonly available for donation. This higher frequency is due in part to the possibility of living donors giving one kidney, leaving them with the other to maintain their renal function. Additionally, the waiting times for kidney transplants can be rather lengthy, during which patients might utilize dialysis to perform the blood-cleansing functions normally carried out by the kidneys.
The need for organ transplants, including those for the liver and heart, often arises from either severe organ damage or failure. However, heart transplants require immediate transplantation after the donor's death, and the lack of viability beyond a short time span makes their transplantation more challenging. The prevalence of kidney transplants is therefore higher due to the greater suitability for donation and transplantation.