Final answer:
The reaction between HCl and PO43- forms the conjugate pairs HCl/Cl- and PO43-/HPO42-, with the favored direction for equilibrium being to the right, towards the formation of weaker acids and bases.
Step-by-step explanation:
The correct conjugate pairs for the reaction between HCl and PO43- are HCl/Cl- and PO43-/HPO42-. When HCl donates a proton (H+), it becomes its conjugate base, which is Cl-. PO43- is the base that accepts a proton, forming its conjugate acid, which is HPO42-. The favored direction for the equilibrium according to the relative strengths of acids and bases is to the right, favoring the formation of the weaker acid and base. Therefore, the corrected option is C. Conjugate pairs: HCl/Cl-, PO43-/HPO42-. Favored direction: Right.