Strongest oxidant among the following is

For Tl +1 oxidation state is more stable than +3 oxidation state .Therefore Tl+3 taking 2-electrons and converting to Tl+1.Hence Tl+3 acts as strong oxidising agent(electron gainer).Stability order of +3 oxidadtion state:B+3 > Al+3  > Ga+3  > In+3  >  Tl+3+III oxidation state (higher oxidation state) stability decreases from top to bottom in a group .Stability order of +1 oxidadtion state:B+1 < Al+1  < Ga+1  < In+1  <  Tl+1→ +I oxidation state (lower oxidation state) stability increases from top to bottom in a group .