The correct answer is H+.
Secretion from the peritubular capillary network into the convoluted tubules is a process called tubular secretion. This is an important process for both waste removal and acid-base balance. The secretion step is usually used to remove drugs, toxins, and poisons, or other molecules such as potassium (K+) and hydrogen (H+). The secreted molecules come from the peritubular capillaries and pass through the interstitial fluid before going through the wall of the tubule into the lumen of the tubule.
