Calculate Ecello corresponding to each compound undergoing disproportionation reaction. The reaction for which Ecello comes out +ve is spontaneous.
HBrO→Br2Eo=1.595,SRP(Cathode)
2HBrO→Br2+BrO3−
Ecello= SRP (cathode) - SRP (Anode)
=1.595−1.5
=0.095V
Ecello>0⇒ΔGo<0 [spontaneous]