Which is the most prevalent species in a 0.25 m aqueous solution of phosphoric acid h3po4 g?

Answer is: H₃PO₄.

A phosphoric acid is three protic acid, which means that in water release tree protons. Phosphoric acid ionizes in three steps in water.

First step: H₃PO₄(aq) ⇄ H₂PO₄⁻(aq) + H⁺(aq).
Second step: H₂PO₄⁻(aq)⇄ HPO₄²⁻(aq) + H⁺(aq).
Third step: HPO₄²⁻(aq) ⇄ PO₄³⁻(aq) + H⁺(aq).
Species that are present: H₃PO₄, H₂PO₄⁻, HPO₄²⁻, PO₄³⁻ and H⁺, but because the acid dissociation constants are low, very low percentage of acid dissociates.