The Acid Ionization Equilibrium Constant, K_a, Describes Which of the Following

The acid ionization equilibrium constant, K_a, describes which of the following reactions where HA is a generic weak acid) ?

A) HA(aq) +OH sup>- (aq) → H₂O(l) + A^-(aq)
B) HA(aq) + H₂O(l) → H₃O⁺(aq) + A^-(aq)
C) HA(aq) + H₃ O⁺(aq) → H₂ A⁺(aq) + H₂ O(l)
D) HA(aq) + H₂ A⁺(aq) → H₂ A⁺(aq) +HA(aq)
E) H₃O⁺(aq) +A^-(aq) → HA(aq) + H₂O(l)