The Acid Ionization Equilibrium Constant, K_a, Describes the Reaction (Where $\leftrightarrows$

The acid ionization equilibrium constant, K_a, describes the reaction (where HA is a generic weak acid) :

A) HA + OH^- $\leftrightarrows$ H₂O + A^-
B) HA + H₂O $\leftrightarrows$
H₃O⁺ + A^-
C) HA + H₃O⁺ $\leftrightarrows$
H₂A⁺ + H₂O
D) HA + H₂A⁺ $\leftrightarrows$
H₂A⁺ + HA
E) H₃O⁺ + A^- $\leftrightarrows$
HA + H₂O