Final answer:
The kidneys aid in acid-base balance by altering the rate of reabsorption of bicarbonate ions (Option D).
Step-by-step explanation:
The kidneys maintain acid-base balance primarily by altering the reabsorption of bicarbonate ions, which help to regulate blood pH.
Bicarbonate ions act as a buffer in the blood and other bodily fluids, helping to maintain the pH within a narrow range.
When blood pH is too low (too acidic), kidneys reabsorb more bicarbonate to counteract the acidity.
Conversely, when blood pH is too high (too basic), the kidneys excrete more bicarbonate in the urine to lower the pH.
The kidneys also control acid-base balance by excreting excess hydrogen ions.
Hence, the correct answer is Option D. Bicarbonate ions.