Phosphoric acid, H3PO4, will undergo three successive ionization reactions to varying extents in water. What is the balanced equilibrium identified as Ka3

HPO₄⁻² +  H₂O  ⇄  PO₄⁻³  + H₃O⁺       Ka3

Explanation:

Phosphoric acid, H3PO4 is a dyprotic acid which undergoes in these three succesive ionization reactions.

The last equillibrium is conditionated to the Ka3

H₃PO₄  +  H₂O  ⇄  H₂PO₄⁻  + H₃O⁺     Ka1

H₂PO₄⁻  +  H₂O  ⇄  HPO₄⁻²  + H₃O⁺   Ka2

HPO₄⁻² +  H₂O  ⇄  PO₄⁻³  + H₃O⁺       Ka3

It is an acid because it release a proton, to make the [H₃O⁺] rise.

pH of solution will always be < 7

H₂PO₄⁻  and  HPO₄⁻²  are amphoteric compounds, which means that they can work as an acid (release protons), or base (take protons)

it is b that is the answer.