Secretion into the tubular fluid occurs when principle cells in the pct secrete h + into the urine when the principle cells in the dct and collecting ducts reabsorb na + and secrete k + when the cells in the loop of henle become impermeable to water never occurs at all

I believe that secretion into the tubular fluid occurs when the principle cells in the Proximal convoluted tubule secrete H + into the urine.

Tubular secretion is the transfer of materials from peritubular capillaries to the renal tubular lumen; it is the opposite process of reabsorption. This secretion is caused mainly by active transport and passive diffusion.