Values of dissociation constant , ${\mathrm{K}}_{\mathrm{a}}$ are given as follows 

Acid                           ${\mathrm{K}}_{\mathrm{a}}$

HCN                   $6.2\times {10}^{-10}$   
HF                       $7.2\times {10}^{-4}$
HNO₂                 $4.0\times {10}^{-4}$

Correct order of increasing base strength of the base ${\mathrm{CN}}^{-}, {\mathrm{F}}^{-}$ and ${\mathrm{NO}}_2^{-}$ will be