Dialysis is a medical procedure used to perform the function of the kidneys. The kidneys are responsible for filtering waste products, excess fluids, and toxins from the blood. When the kidneys are not functioning properly due to diseases such as kidney failure or when they are removed due to transplantation, dialysis can be used to replace their function. During dialysis, a machine is used to filter the blood, removing waste materials and excess fluids. This process helps maintain the balance of electrolytes and fluid levels in the body, which is essential for overall health. Therefore, option 4 (Kidneys) is the correct answer. It is important to note that dialysis does not perform the function of the lungs, heart, or liver.