Identify the concept involved

Aluminium and thallium both belong to Group 13 of the periodic table.

• Aluminium commonly shows oxidation state $+3$.
• Thallium shows both $+1$ and $+3$, but the $+1$ state is especially stable.

Reason for anomalous behavior of thallium

Thallium has outer electronic configuration: $[Xe]4f^{14}5d^{10}6s^26p^1$

In heavy p-block elements like thallium, the outermost $ns^2$ electrons tend to remain less available for bonding due to poor shielding by inner $d$ and $f$ electrons.

This makes the $6s^2$ electron pair relatively "inert" or reluctant to participate in bonding.

Hence:

• Losing only the $6p^1$ electron gives oxidation state $+1$.
• Losing all three valence electrons gives oxidation state $+3$.

The extra stability of $+1$ in thallium is due to the inert pair effect.

Check the options

• A: inert pair effect — Correct.
• B: diagonal relationship — Not relevant here.
• C: lattice structure — Not the cause of oxidation state variation.
• D: lanthanoid contraction — It influences size and some properties, but the direct reason for stability of $+1$ in thallium is the inert pair effect.

Final answer

Therefore, the correct option is: $\boxed{\text{A: inert pair effect}}$

Comparison with stored correct answer

Stored correct answer: A

My derived answer: A

They agree.