Choose the correct stability order of group 13 elements in their +1 oxidation state.

Strategy: Rank Group 13 elements by stability of their +1 oxidation state.

Step 1: Inert Pair Effect The $+1$ oxidation state requires retaining the $ns^2$ pair (inert pair). This tendency increases down the group due to the inert pair effect (relativistic contraction of $ns$ orbitals, poor shielding by d/f electrons).

Step 2: Stability Order

• Al⁺: least stable (Al strongly prefers +3)
• Ga⁺: slightly more stable than Al⁺ (but Ga still usually +3)
• In⁺: more stable; In(I) compounds known
• Tl⁺: most stable — Tl(I) is the predominant oxidation state

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

$\boxed{\text{Answer: (A) }\ce{Al < Ga < In < Tl}}$