Question

Which of the following is the conjugate acid to a base in these options :

1) PO4^-3
2) H2O
3) H3PO4
4) H2PO4^-

Answer 1

I don't understand the question.

1 is the conjugate base of HPO4^2-.
2 is the conjugate base of H3O^+.
3 is the conjugate acid of H2PO4^-
4 is the conjugate acid of HPO4^2- and the conjugate base of H3PO4.