Four species are listed below:
$\text{ (i) }{\mathrm{HCO}}_3^{-}$  $\text{ (ii) }{\mathrm{H}}_3{\mathrm{O}}^{+}$
$\text{ (iii) }{\mathrm{HSO}}_4^{-}$    $\text{ (iv) }{\mathrm{HSO}}_3\mathrm{F}$
Which one of the following is the correct sequence of their acid strength
 

HSO₃F is strong acid. Also H₂O is neutral, its conjugate base is stronger and thus, H₃O⁺ is stronger acid than HCO₃^- and HSO₄^-. Also HCO₃^- is stronger base of conjugate weak acid H₂CO₃ whereas HSO₄ is weak base of strong conjugate acid. Thus, HSO₄ is stronger acid than HCO₃^-.