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. 

Rna polymeras should attache to the promoter.