The correct option is D ionic and pH balance in body fluids
Urine formation has three main steps - ultrafiltration, reabsorption and secretion. While ultrafiltration is carried out by glomerulus, reabsorption and secretion is carried out by renal tubule.
Cells of renal tubule secretes substances like H+, K+ and ammonia into the filtrate. Tubular secretion helps in maintaining ionic and pH balance in body fluids.