Which of the following is the best oxidising agent in aqueous solution, owing to the stability of the lower oxidation state of the metal?

Down group 13 the inert pair effect makes the $+1$ state increasingly stable. Thallium(III) therefore readily accepts two electrons to become thallium(I), so $\mathrm{TlCl}{}_3$ is the strongest oxidant of the set. Aluminium has no stable $+1$ state at all.