Write an equation that represents how dihydrogen phosphate ion (H2PO4-) behaves as an Arrhenius acid. ( Be sure to include the correct charges for ions.)

Answer 1

An Arrhenius acid is one that releases(provides) H^+ when placed in solution.
H2PO4^- ==> H^+ + HPO4^2-

I see some sites where the definition has been modernized to include H2O like this
H2PO4^- + H2O ==> H3O^+ HPO4^2-
however then one must explain that the hydronium ion actually is the H^+ (really the other way around); I don't think Arrhenius knew anything about the hydronium ion.

Answer 2

The second one you had described turned out to be the answer.