Answer is: the weakest acid is with Ka = 1.5 × 10⁻¹⁰ M.
Ka is the acid dissociation constant, which quantifies the extent of acid dissociation in water.
The lower is the acid dissociation constant, the weaker is the acid, because concentration of hydrogen ions (H⁺) or hydronium ions (H₃O⁺) is very low.
For some acid, chemical reaction in water: HA(aq) ⇄ A⁻(aq) + H⁺(aq), acid dissociation constant is: Ka = [A⁻]·[H⁺] / [HA].