The decreasing order of relative stability of $+1$ oxidation state of group $13$ elements is

The correct option is A

$\mathrm{Al}<\mathrm{Ga}<\mathrm{In}<\mathrm{Tl}$

Oxidation state: The oxidation state, also known as the oxidation number, is the hypothetical charge of an atom if all of its connections to other atoms were entirely ionic.

Inert pair effect:

• The inert-pair effect is the tendency of the two electrons at the outermost atomic s-orbital in post-transition metal compounds to stay unshared.
• The inert pair effect is caused by a drop in bond energies as one moves down a group.
• The ns electrons do not get much more inert (lower in energy) as one lowers the periodic table groups.
• As we move down the group, the inert pair effect becomes stronger due to an increase in atomic size.
• As a result, the greatest oxidation state's stability drops while the lowest oxidation state's stability improves.
• In the $13\mathrm{th}$ group, the decreasing order of relative stability of the $+1$ oxidation state is
• Aluminium($\mathrm{Al}$) $<$Gallium($\mathrm{Ga}$) $<$ Indium($\mathrm{In}$) $<$ Thallium ($\mathrm{Tl}$).

Thus, the correct option is (a).