The cathode is the electrode where the reduction occurs.
The half-reaction that has the highest tendency to occur is that with the highest positive reduction standard potential, Eo.
So, it is Br2 + 2e- ----> 2Br- with Eo = + 1.07 V
Answer: Br2 + 2e- ----> 2Br(-) , Eo = + 1.07 V
