Chlorine undergoes disproportionation reaction in alkaline medium. It is the simultaneous oxidation-reduction. Chlorine is simultaneously reduced to chloride ion Cl− and is oxidised to 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,
⇒Cl2−2OH−⟶Cl−+ClO−+H2O
Hence the values of a,b,c,d are 1,2,1,1.