Which property of the acids is observed when they are added to water?

When acids are added to water, they dissociate to form hydronium ions (H₃O⁺) and hydroxide ions (OH^-). This phenomenon is known as ionization.

$\mathrm{HA} + {\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+} + {\mathrm{OH}}^{-} + {\mathrm{A}}^{-}$

Thus, the correct answer is 'Ionization'.