The relative stability of +1 oxidation state of group 13 elements follows the order

We begin by recalling that the common or “normal” oxidation state for the elements of group 13 is $$+3$$, because every atom in this group possesses the outer-shell configuration $$ns^{2}\,np^{1}$$, giving three valence electrons that can, in principle, be lost during oxidation.

However, as we descend the group, another important concept—the inert pair effect—becomes more and more significant. The inert pair effect can be stated as follows:

“For heavier p-block elements, the two $$ns$$ electrons (called the ‘inert pair’) tend to remain non-bonding due to relativistic contraction and poor shielding by intervening d and f orbitals, making oxidation states two units lower than the group oxidation state progressively more stable.”

Applying this statement, we see that for group 13 elements the oxidation state $$+1$$ (which is lower by two units from the normal $$+3$$) should become increasingly stable down the group, because the inert pair of $$ns^{2}$$ electrons is less readily removed.

In other words, the stability trend should be

$$\text{B} \; < \; \text{Al} \; < \; \text{Ga} \; < \; \text{In} \; < \; \text{Tl}$$

Since boron almost never exhibits $$+1$$ and is not included in the options, we restrict ourselves to Al, Ga, In and Tl. Removing B from the series gives

$$\text{Al} \; < \; \text{Ga} \; < \; \text{In} \; < \; \text{Tl}$$

Thus, the relative stability of the $$+1$$ oxidation state increases in precisely this order.

Comparing with the choices provided, this matches Option D.

Hence, the correct answer is Option D.