The correct option is D Counter current mechanism in Henle's loop/vasa recta
Counter current mechanism in Henle's loop and vasa recta helps to maintain concentration gradient in the medullary interstitium. Presence of such interstitial gradient helps an easy passage of water from the collecting tubule thereby concentrating the urine.
PCT helps in maintaining pH and ionic balance of the body fluid by the secretion of H+, ammonia and potassium ions and reabsorbing 70-80% of electrolytes and water.
Collecting duct helps in reabsorption of water to produce concentrated urine and in maintaining osmolarity. It also plays a vital role in maintaining pH and ionic balance in blood by secreting H+ and K+ ion.
DCT reabsorbs Na+ and water along with HCO–3, maintains pH and sodium- potassium balance in blood by selectively secreting hydrogen ions, potassium ions and ammonia.