During urine formation, the tubular cells actively secrete H${}^{+}$, K${}^{+}$ and ammonia into the filtrate, helping maintain ionic and acid-base balance.

• PCT (proximal): selective secretion of H${}^{+}$, ammonia and K${}^{+}$ into the filtrate; large-scale reabsorption of useful substances.
• DCT (distal): capable of reabsorbing HCO${}_3^{-}$ and selectively secreting H${}^{+}$, K${}^{+}$ and NH${}_3$.

The diagram in option (3) matches this pattern correctly.