Which one of the following elements does not form triiodide on reacting with iodine?

Boron show consistently +3 oxidation state in its compounds while rest of the elements show +1 as well as +3 oxidation states. The +1 oxidation state becomes more and more stable as we move down the group, i.e. $$ \mathrm{Al} < \mathrm{Ga} < \mathrm{In} < \mathrm{Tl} $$ This is due to the presence of intervening $d$ - and f-orbitals which do not shield the nucleus effectively. Consequently effective nuclear charge increases, that holds the ns-electrons tightly so that they do not participate in bonding (inert pair effect). Due to inert pair effect, $\mathrm{Tl}$ in +1 oxidation state is more stable than that of +3 oxidation state.