Aluminium is usually found in +3 oxidation state. In contrast, thallium exists in +1 and +3 oxidation states. This is…

🧠 Heavy p-block elements (Period 6) keep their $ns^2$ pair locked in the core — the inert pair effect As you go down the p-block, the $ns^2$ valence electrons become reluctant to bond. They "stay inert". This is because the heavier element's $ns$ orbital penetrates deeper, gets stabilized more, and is poorly shielded by the inner $4f$ and $5d$ electrons. The result: heavy elements often skip using their $ns^2$ in bonding and show oxidation states that are 2 less than the maximum.

🗺️ Apply to Group 13 Al (Period 3): uses all three valence electrons ($3s^2\,3p^1$) → only $+3$ state. Ga (Period 4): mostly $+3$, slight $+1$ tendency. In (Period 5): $+3$ and $+1$ both seen. Tl (Period 6): $+1$ is the dominant state. The $6s^2$ pair stays inert.

So Tl shows both $+1$ and $+3$ because of the inert pair effect.

⚠️ The trap Option (c) Lanthanoid contraction is a related effect — but it explains why Tl is small for its period, not why Tl prefers the $+1$ state. Lanthanoid contraction affects atomic SIZE; inert pair effect affects which OXIDATION STATES appear. Don't confuse the two.

$\boxed{\text{Answer: (d)}}$