Chlorine undergoes disproportionation reaction in alkaline medium. It is the simultaneous oxidation-reduction. Chlorine is simultaneously reduced to chloride ion ${\mathrm{Cl}}^{-}$ and is oxidised to  ${\mathrm{ClO}}^{-}$  ion. Halogens (e.g. Cl) have a strong tendency to accept electrons, so they act as strong oxidising agents.

The reaction is as follows,

\(\Rightarrow \mathrm{Cl}_2 - 2 \mathrm{O} \mathrm{H}^- \longrightarrow \mathrm{Cl}^{-}+\mathrm{ClO}^{-}+\mathrm{H}_2 \mathrm{O}\)

Hence the values of a,b,c,d are $1,2,1,1.$