Answer
d. tubular reabsorption of sodium
Work Step by Step
Tubular secretion of hydrogen is directly linked to tubular reabsorption of sodium.
In the tubules $Na^2HPO_4$ reacts with $H^+$, exchanging a $Na^+$ for a $H^+$, and resulting in $NaH_2PO_4$. The monobasic sodium phosphate is then eliminated in the urine while the $Na^+$ reenters the bloodstream.
