Key concept: inert pair effect

For group 13 elements, the common oxidation state is $+3$ because their valence configuration is $ns^2 np^1$.

The $+1$ oxidation state arises when only the $np^1$ electron is lost, while the $ns^2$ pair remains non-bonding.

As we move down the group, the inert pair effect increases, so the $ns^2$ electrons become less willing to participate in bonding. Therefore, the stability of the $+1$ oxidation state increases down the group.

Apply this to group 13 elements

Group 13 elements here are: $\text{Al},\ \text{Ga},\ \text{In},\ \text{Tl}$

Due to increasing inert pair effect down the group: $\text{Al}^{+} < \text{Ga}^{+} < \text{In}^{+} < \text{Tl}^{+}$

Check the options

• A: $\text{Al} < \text{Ga} < \text{In} < \text{Tl}$ ✅
• B: $\text{Tl} < \text{In} < \text{Ga} < \text{Al}$ ❌ reverse order
• C: $\text{Al} < \text{Ga} < \text{Tl} < \text{In}$ ❌ places Tl less stable than In
• D: $\text{Al} < \text{Tl} < \text{Ga} < \text{In}$ ❌ incorrect placement

Final answer

The correct stability order of group 13 elements in the $+1$ oxidation state is: $\boxed{\text{Al} < \text{Ga} < \text{In} < \text{Tl}}$

Comparison with stored correct answer

Stored correct answer: A

My derived answer is also A, so they agree.